I think you misunderstand me. When I say “sample pack of unknown provenance,” I don’t mean “CODRIPS.ZIP from HOTGARBAGE.IO.”
I mean a sample pack that claims to be royalty free, but it’s sold for $49 from some company you never heard of. E g, you’ve done enough due diligence, but there’s always a chance that someone you rely on is lying.
When it comes to “transformative use,” there totally is fair use when something is transformed enough, and such works do not need original permission.
What is “enough?” What is “transformed?” If someone really cares, it has to be settled in court, which is a place nobody wants to be, no matter whether they’re right or wrong.
So, the main goal is to not get to court.
How do you not get to court?
By creating a sound that doesn’t sound like someone else’s sound. If it sounds significantly different, it’s a new sound, and if someone still somehow complains, if you can show diligence, then you’re in a pretty good position. (Someone might still complain for reasons of being mistaken or just plain lying, too. And if they have more money/lawyers than you, it might be cheaper to settle than the prove you’re actually-right. Nothing is certain! Some company is filing claims about any video with “typing keyboard noises” on YouTube. They might be wrong, but they’re still a nuisance!)
That sounds like exactly my advice, except my additional concern is: “what happens when that SFX bundle was made by the bad guy?”
If you further process the sounds, then you’re in a better place with less risk.