To be very frank, I think many people who claim that their music is completely royalty-free or public domain or whatever do not think about whether all parts of the module
really are (they just think about the composition but not the sample). Many of those modules declared to be in the public domain use ST-01 samples for example, and those samples are taken from famous 80s synthesizers, among other sources. And of course this sample disk was sold alongside a commercial product (Ultimate SoundTracker), and while it has been long discontinued, most people using those samples never paid for that product.
Is it likely that anyone will get sued for using them? Again, no, it's not likely at all that this would ever happen. But if you ask for the legal status of those samples, it's simply impossible to give a clear answer. If being 100% certain that all material you use is 100% royalty-free and legally "clean" is of importance to you, you cannot use these sources. Most people don't go that far and are happy with living a legal grey area that probably noone ever got sued over, though.
More interesting answers on this topic are provided here, especially those answers by Ceekayed are worth reading:
https://modarchive.org/forums/index.php?topic=4602.0
