Is it possible to make an mp3 player with sponge api? xd
Yes it is.
You can include custom songs in the resource pack, and send commands to play them to the client.(assuming the client accepts the resource pack)
Additionally, there was talk of a music player plugin that would play in your browser when you click a link, and have the sponge server remote control it.
In addition to @ryantheleach’s suggestion, a plugin can be developed that plays .nbs files in-game. I believe @clienthax has a utility library for nbs files.
Adding on to @pie_flavor’s comment, here is the plugin by @clienthax that can play music in-game: MusicBox [0.4] [Api 4 + Api 5].
Thats NBT only i meant for .mp3 i alrdy knew about that one xd just asking for mp3 one 
Like @ryantheleach said, you would have to put them in a ressource pack. Maybe it’s a slightly other format(ogg, wav, don’t know)