If the spots on your demo were not done for air, label them as such, even if you use really small letters. Suppose one of the spots on your tape was written and produced by the person listening to it. They are sure to know that you're not the person who originally recorded it, but will assume you're trying to pass yourself off as the original. That could be extremely embarrassing. Don't get caught in that position.
People have memories like elephants for negatives.
While most any recording studio can record a VO demo, most don't know how to produce them. However, if you have a friend at a radio station, you might want to check there. If you work at a radio or TV station, you'll get off cheap. The cost will depend on where you go. Some studios charge a flat rate while others have an hourly fee.