Over ten years ago I worked at a startup company that no longer exists today. Recently I found some of courses we developed on a CD Rom. They were copied by another employee who gave them to me years ago. Since the company no longer exists in any shape or form in the US, is it legal to use those courses? If I do, should I acknowledge that they used to belong to that company in the copyright area? Thank you for your help and input.
When a company goes defunct, the assets typically do not just enter the public domain. Someone owns the property. Whether it's the original owners or, potentially, creditors in the event that it died through bankruptcy or other legal actions.
You need to make an honest effort to track down the previous owners and discuss with them what you want to do. They should be able to tell you who owns the rights so that you can negotiate to acquire them. Probably for a nominal fee.