As the course Preamble explains, that unit must be at least 16x speed to avoid problems. The CD-ROM drive lens of your computer needs cleaning or readjusting. This is not an infrequent occurrence. There are products that will clean the lens of the unit. As with any other CD-ROM reader or player, maintenance is necessary.
Repair shops are full of machines whose only problem is lack of maintenance.
You have too many applications open, and, therefore there isn't enough memory available to the system when you play a music example. The RealPlayer, therefore, starts competing for system resources with other programs. Don't try to run the course while having your email client, word processor, Excel spreadsheet, and maybe a game or two open at the same time. It is obvious that, under those circumstances, the sound is going to skip or stop. If you have an older computer, or one that doesn't have enough memory, you should stop the example you are listening to before clicking on the next one.