It's a problem that has haunted the LMMS sample base for a while - no-one now knows where those samples came from, nor what license they were released under, and no-one (sadly) seems to be bothered to remove them and replace them with known-license versions.
This said, I don't know why Nils is struggling. There's FreeSound - http://www.freesound.org/ - which has mammoth quantities of samples of everything from real drumkits and instruments to environmental effects and found noises (and I'm only including 'real' instruments and samples here - there's a lot of synthesised samples there too). There's the OLPC Free Sound Samples library - http://wiki.laptop.org/go/Free_sound_samples - which includes lots of libraries of professionally-recorded, full quality instrument samples. All of those are available under remix- and rework-friendly Creative Commons licenses.
High-quality free instrument synthesisers are a bit harder to find. But you can't go past ZynAddSubFX, which is also built into LMMS, for a huge variety of synthesized natural and artificial instrument patches.
If he's looking for SoundFont (.sf2) files, then I think that's a more difficult ask. But to me that's like saying there are no Creative Commons-licensed 78RPM records: it's ignoring the forest for the trees.