I've found the media_attachment model:
https://github.com/tootsuite/mastodon/blob/master/app/models/media_attachment.rb

Defining audio types there looks straightforward in enough. What I have no idea how to even begin doing is making it render an <audio> tag in the timeline for inline playback.

Forgive me, I am not a developer.

#mastodev

@nivex

so, I've dug into this one in the past, to the point of implementing audio uploads on the glitchsoc fork. I ended up transcoding the audio to a video format to get around the fact that tootsuite mastodon hasn't whitelisted the <audio> tag and without that whitelisting none of the unmodified mastodon instances would actually render the tag. I have attempted to get tootsuite to let us just use the audio tag, but Eugen has aesthetic arguments against it so it's not gotten anywhere. Please feel free to let him know that you are yet another user interested in this and maybe someday there'll be enough of us for him to give in.