I'm not sure how changing the music works in minecraft but I think a great idea would be a 2nd type of music which you can change to via a setting in the audio settings area.
I'm not suggesting thumping EDM or whatever. Something like electronic ambient music etc. where pixelmon players who make music, could contribute (or find someone else... no love).
And for customisable. You could add a music folder in the pixelmon-beta or .minecraft folder kinda like forge adds the mods folder. Then to add your own music, you create a folder inside that folder named whatever you want.In the audio settings in minecraft, you could click the music genre setting and it would change from classic > ambient > [custom].
I have no idea if this is simple or difficult or what... kind of a noob (there's no "kind of")