Congrats on your new slippery slope haha

ike I fire up a docker image which plays music (if that's even possible?) it has to have access to the disc, sound drivers, maybe interactive stuff etc on the host PC right?

So the main things you'll want to read up on for that are mounts. Mounts will let you attach files and folders from the host computer into the docker container that it sees as if it's inside the container.

A lot of docker apps will run a web host, so instead of accessing them like a normal application, you load up the website that's located at the IP address, and the exposed port. Then just like running Netflix or anything, it already has access to local sound and video devices through that

This also means that you can open them up to other computers/devices on the home network... so your phone could load it up and play music or your windows PC could, and it's all served from that docker container

If you're interested in hosting media, you could look into Plex or jellyfin, they are media servers that can stream self hosted videos, music and photos over the network.

There's a lot of other options that are more specific, and what's right for everybody else might not be right for you so it's worth playing around with various options