Thanks for your feedback. I do not live in the US, by the way.
I'm not too concerned about using instrument samples, which as Saga Musix put it are in "a bit of a grey area", since in a mixed state, combined with other instruments, various pitches and filters etc. it's hard for anyone who doesn't have the source file available to prove that "this and that specific sound" is specifically theirs. I still, for own peace of mind, try to have as few grey areas as possible, though, and no distinctly black areas, i.e. instrument samples that are so unique that they can be unambiguously identified, which I haven't specifically licensed for use.
In that respect I'm concerned about some electric guitar samples I use in my latest tracks, which I sampled off of a YouTube video (segmented into single guitar notes). Very hard for anyone to identify, of course, but I know about it. There were some good new samples in the WaveWorld pack, and I've also found some on FreeSound, labled public domain, which I aim to replace them with, though.
Voice samples, on the other hand, are much less of a grey area, very much more leaning towards black, as they can mostly be easily identified by their respective owners, regardless of whatever other stuff there is in the mix. For those purposes I'll have to use solely samples I've officially licensed, samples that are unambiguously regarded to be in public domain, or samples I've recorded myself, before I try to release it.