For the record. I asked for help on the creative commons mailing list.
Moreover, I think that to be valid, the license should be specified at
least in the id3 tags, and/or it should come with the download. When you
download my albums from Jamendo, for instance, the license is specified
both in the id3 tags and in a text file included in the zip archive. At
Opsound the downloads pointed to the tracks hosted on my own server,
which had no licensing information.
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