"Downsizing" an mp3 is what we call transcoding, and it's bad. An mp3 (or the better ogg vorbis) works (basically) by discarding parts of the audio that you won't hear. Doing that will discard parts of the mp3 that already has a bunch of parts discarded and this makes it sound bad.

I do something like what you're looking for, but it's all through a bunch of custom scripts and crap that I made. I buy flac, encode that to ogg (which sounds better than mp3s at lower bitrates), those oggs get synced to my phone, then later I organize those oggs to my collection.

I think you'll have to either accept these large mp3s or start a lossless collection.

I would sooner suggest that you find a decent hosting service of free storage or just shove a decent sized microSD into your phone. Lossless music is the way to go. I started doing the mp3 thing quite a long time before most of the world even knew what they were, having been on the pirating scene early in the 90s. So back then I got most of my music in highly compressed mp3. But in the recent years I’ve been slowly replacing all of it with lossless format and it sounds SO MUCH better.

So while I hope you get your answer, should you choose to go that route as you’ve tentatively planned, I’d just suggest you pause a moment and rethink whether it’s the route to go. MicroSD chips are very cheap, and if you have an iPhone so it’s not an option, you could get a cheapie Android phone with a slot and use that for music. You can get a nice small device several years old online for like $15. I even have quite a few I could provide you with one if you wanted.

This might be a bit overkill for what you want but you could try using a selfhosted music server like navidrome and streaming to your phone. I use symfonium on on phone which can be configured to request the streamed music to be transcoded to a smaller size for streaming from mobile network or for caching it on your phones storage for offline listening.

Given that symfonium supports a lot of self hosted media providers from which to pull, you could also try sharing your music locally using samba. I'm not sure if the transcoding still works in that case though (it would obviously have to be done on your phone)

For lower bitrates, I'd suggest using a different codec than MP3. Opus is really solid, and at 128 kbps it will probably get you quality similar to MP3 at 192 kbps. Or you could go lower, and 96 kbps with Opus will be similar to MP3 at 128 kbps. I don't know an app that will do it automatically, but the CLI tools are really simple to use: you point them at the FLAC and tell it the target bitrate and that's it.

Alternatively, if you have access to a macOS machine, their AAC encoder is really good and likely superior to any MP3 encoder at equivalent bitrates.

It's not really an "App" but tools like ffmpeg or sox or lame can do that no problem. It might take a while to convert your entire collection though ... but depending on the size might just take a night or, few nights.

If you have a ridiculously large collection and do want it "on demand" you could also use e.g. inotify to monitor directories, e.g. ~/Music/ForPhone/ so that any file added to that directory gets converted.

FWIW I'd use a phone with a microSD card as those days one can get a 1To for less than 100€ so probably no conversion needed even for a large collection.

Edit: based on a recent conversation I'd try transcoding capabilities of LMS https://github.com/epoupon/lms cf https://lms-demo.poupon.dev/settings from their demo instance

Well, it's not great to take an already compressed audiostream like 320kbps mp3, and then compress it again. Use your FLACs if possible. Then, I'd recommend that you use .ogg - they give better compression (smaller files), but with better sound than mp3...

There's a lot of apps out there, but AFAIK not anyone who does this automatically for you.

128kbps mp3s are terrible

Have you considered self-hosting Plexamp? It supports lossless quality and lets you both steam and download your music at any bitrate you want (even 128 if you really have to, but fuck me that hurts to type.

That's what I do, and before long trips with spotty service I'll download a dozen albums or so in flac so I've got a decent lossless selection for flights/etc

Handbreak could be used for that and automatically run for set directories if i remember correctly.

Navidrome does that. You have to setup a PC, or a raspberry Pi with navidrome, and then use a client like Symfonium (costs $5, not open source, but it's the best subsonic client out there), and tell it to automatically downconvert music when played via the phone. I have a Raspberry Pi 3B+, with just 1 GB of RAM, running navidrome. DietPi + navidrome (which is installable directly via dietpi's software selection), together they take just 80-120 MB of RAM!

I had Jellyfin before that, and Emby, and they were dogs. 1 GB of RAM was not enough for them, they'd swap with an additional 200-300 MB of RAM. And they were slow with large music libraries too. Navidrome/Subsonic don't have such issues. Big music libraries are handled fast with their db/engine.

If you prefer to not use a server, there are encoding shell scripts that do batch-encoding: https://github.com/caleis/flac2mp3/blob/master/flac2mp3.sh

Not sure if it's exactly what you're looking for - but subsonic/airsonic/etc. can do on-the-fly conversions when downloading to mobile devices. There are several mobile apps available that will cache files locally as well.

Can you stream from your pc? I set up Jellyfin and Tailscale, and all my music (and movies and tv shows) are on my computer, accessible from anywhere.

not automatic but simple enough and very efficient; I'd use ffmpeg

DbPowerAmp has a bulk converter tool. It's proprietary payware but cracked versions are available.

I don't know about automatic, but I also have music locally on my phone after moving away from streaming services.

I use fre:ac to convert from FLAC to MP3, and then just save it in a separate folder. Then when I'm done I move the folder to the phone and delete it from the computer so I just have FLAC on my computer.

Not automatic, but that's how I usually do it.

You can use Plex to stream music from your own server. The mobile app is called Plexamp, and it’s pretty great.

Not automatic (I think) and a bit clunky but the Strawberry music player does have a transcode feature so you could select music files and transcode them a certain way output to another folder. It's not something I ever do but I did a quick test to a USB drive and it seems to work okay. It's an option if you opt to use a gui to click through.

OTOH if you're happy using the terminal and/or scripting then ffmpeg may be a better bet.

PS - Strawberry does have a panel where it lists "Devices" and maybe your phone could show up there and the transcoding would work a bit more automatically, wasn't able to test that here.

Might not be the most convenient option depending on your personal use-case, but have you considered a dedicated audio device? I personally got a refurbished LG V30 because I came across a YT vid while looking for cheap options for having a dedicated "mp3" player to keep archived/favorite episode of podcasts, audiobooks, and music. Seems there is a decent fanbase of folks that love the 3.5mm DAC that phone has (of course has Bluetooth too). It also has microSD slot. Got a 512GB card (can use up to 2TB which I might do at some point). Most of the music I have on it is flac where possible. I keep it offline and just transfer files via USB, but could use wifi to sync with PC if I wanted to at some point. Shit lasts a pretty long time with wifi/bluetooth off only using the headphone jack (helps that the battery was replaced by the refurb).

There are also lots of cheaper Android-based players (I got one before I came across the vid on the V30) but can have some amount of malware and no easy access to their firmware or communities that can advise custom ROMs to safely flash something clean and/or newer. When I got my no-name device I just side-loaded apks to avoid giving access to my Google account (though I plan to keep my V30 offline after I got the last updates for the OS and LG apps). I just wanted to have something that I only use for local audio and just keep it in my car or backpack and have access to the three apps I like (Musicolet for music, Podcast Addict for podcasts, and Smart AudioBook Player for my audiobooks).

https://beets.io/

This is what I use

Just use a ~500GB SD card, but it costs around 40€.