Just about any computer built in the 21th century is capable of being used as a simple file server, which is what we're discussing here. Even a single core Pentium machine from the 20th century would probably work just fine. Reading a file from disk and passing it on through whatever interface that one prefers isn't very demanding of CPU resources, neither is audio file decode and playback.
The RPi is merely a very versatile single board computer that applies to DIY. Others use mini PC, NUC, laptop, and desktop machines for the task. Even Android mobiles can be used as such - I'm not kidding...