Looks like my question about samples is settled, but what about software synthesizers?
They might be licensed under GPL, but the software license is not
transferable to audio output from the software. I should have the right record the output
from synthesizers and waive it under CC0.
So if I make music only with included software synthesizers, I should be able to waive both
render and project file to CC0, right?
Now, how do we draw a line between sample and software synth?
Is BitInvader a pure synth? What about instruments included with ZynAddSubFX?
Here's a list of foss instruments and notes on licensing:
https://hilbricht.net/foss-sampled-instruments.html
Also, musicbear's reply made me a bit confused. He said that distributing project file
under CC0 would also distribute samples which is not allowed. But, a project file
does not contain official samples from lmms, so I wouldn't call that redistribution of samples.
(I might be wrong with the text I wrote, tell me if that is the case)