Neither I'm a lawyer, so we're even. :) The requirement for complete corresponding source files isn't in the license per see, even if the license is indeed GNU GPL 3+ (let's assume), --- because as you said, there is possibility of the interpretation you just made ---, but the requirement I spoke of is in the Definition of Free Cultural Works, in [[http://freedomdefined.org/Definition]], which is what defines "free culture".
About sound recordings: I'd say that the music composition being recorded has complete and corresponding source which is the score. However, in music improvisation, since it has no score, then the recording itself is the source. For other points raised by you but not answered in this message, see the Definition of Free Cultural Works in the reference I just presented.
