It depends on the CD, but unless one of the CDs was specifically released in the public domain, there's no way they're public domain. I'd say many/most sample CDs are royalty-free (for those that actually bought them). If you're super paranoid about using these sample CDs commercially for a video game, consider:
- Tracking down the original license agreements for the sample CDs you want to use.
- Contacting the original publishers to discuss your intended use and see if you can obtain the necessary rights. (aka throw money at them)
- Considering using royalty-free sample libraries that explicitly allow use in video games.
However, these sample CDs are from the 90s and the samples and recordings are spread far and wide across the internet already, in various sample packs, tracks, and otherwise. If you ask me, you face zero risk of copyright holders pursuing you over torrenting and using these now ancient samples in your own commercial work. Take and use whatever you want- I promise no one gives a shit.