I'd use filenames locally and return URLs to media. Reference the music and image files rather than stuffing them into the database. Store names, filenames and all the metadata you think makes sense (artist, album, genre and all sorts of extra tags) in the music file table.
If presenting a web API, JSON is perfectly fine (and most common). Any web frontends are likely to be JS anyway, so use a natively easy to use format. The built-in packages in the Go distribution can get you all the way, but if you like a little extra help with routing, Chi is a decent option: