Raspberry pi zero is the cheapest server I ever built. It ran like crap but it was fine for hosting any audio. My idea was to just have it with a usb drive plugged in hosting away but then I figured I may as well just use my Mac because it was on anyway.

Not quite as self hosted as other things here but, You could look into plex. They have made Plexamp free to use now and it is actually pretty good. Has some good features. And more importantly has a phone app that works.