Termux will accept files "shared" to it, and pass the full filename to ~/bin/termux-file-editor

So, if it's the only thing you plan to use Termux for, you could make your ~/bin/termux-file-editor something like:

#!/bin/bash
ffmpeg -i "$1" -b:a 320k "$1.mp3"

...and then never think about it again, just share files via Termux and it'll re-encode them for you. :)

or, if you don't specifically need CBR and just want ffmpeg to output on its highest quality:

#!/bin/bash
ffmpeg -i "$1" -q:a 0 "$1.mp3"

I'm not sure of the exact syntax, but hope this at least helps point you in the right direction :)