I realise the software for UFO will be released under the terms of the GPL, however it does not really apply to music, being a software license, does it? (I'm not really a lawyer or anything so this stuff is slightly over my head).
Wouldn't something like the creative commons license be more appropriate for covering works of sound and music used in the game?
Is your project compatible with Creative Commons? With CC, you are basically free to use someones stuff as long as you give credit for it.
Also, if you support the CC, then there are literally hundreds of thousands of high quality recordings and samples available on the web which we could then use with the only penalty being that somewhere in the documentation we'd have to list the "Sound Contributors" and provide a link to the CC license page.
This is a summary:http://creativecommons.org/licenses/sampling+/1.0/
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(here's the legal code)http://creativecommons.org/licenses/sampling+/1.0/legalcode