SPC is the file format that is used by game music emulators to play back SNES music. MP3 isn't recommended anymore because it can't be looped and has a large filesize.
There's a program in the SMBX2 folder called add_musplay_assoc.bat - run that it'll let you set up a program called
PGE Musplay that will be used to play back SPC files (as well as a few other emulated formats). If you're dead set on using a digital format instead of an emulated one, PGE Musplay can also record SPCs as WAVs. Alternatively,
Audacious (the music player that I use) is also able to play SPC files.
If you want more SPCs (such as to use in a level or for your listening pleasure), I get mine from
Zophar's Domain and
SMWCentral.