It's nearly impossible to collate the various experiments of 20th century European composers into a coherent narrative. As classical music has soldiered on in the face of world wars, pop music and modern art, the methods, the forms and the sounds that composers work with have become wildly divergent. Folk music, electronic music, birdsong: all of these elements and many, many more have become tools of the trade for avant-garde composers the world over. The Naxos label has recently dubbed this music by European mavericks — and their American counterparts — a form of "Sonic Rebellion." It's as good a name as any for the variegated and unconventional sounds you'll encounter here. Dig in.