I saw someone request it in 2012, but people said there was the ipod mod. Which I guess works but there's two problems.
One, the ipod is very over powered. So if you wanted on a public server for people to be able to play music wherever they went, they'd also be able to do a lot of things you'd probably not want them to do.
A portable music player with support for mods that add their own music and isn't over powered like the ipod would be very helpful.
Sadly, a lot of these mod requests don't get fulfilled due to the sheer number of them and/or the lack of java modders looking to make a mod out there. I dare say, I may have to get into java coding myself, for I REALLY want this.Oh even if no one is willing to I probably won't. I can only hope someone will see this and be willing to make this mod.