There isn't really a standard 'source code' format for music, so it would be difficult to release music as open source.

At the risk of nitpicking, there really are ways to release music essentially as open source. You're right about no standard format, but there are many useful formats to release music in for editing purposes. For example: sheet music, unmixed audio tracks, MIDI files, unfiltered samples, synthesizer sound patches, etc. I imagine "open source" music being this sort of raw, uncomposed data which is much more remix-friendly (remixing being the only analogue to source patching that I can imagine).

For that matter, there's no "standard format" for computer source, either. I have source code in C, C++, perl, Python, Java, etc.