Thanks for both suggestions... and sorry for not responding sooner, but have had no chance to get back to looking at this until now.
I updated the drivers and cleared the proxy files as suggested. Unfortunately, I still have very bad audio on the slideshow. I also tried using a different song, and still the same. Just to be sure, I have played the songs in the Edit Audio Files section and they both played fine.
I output the slideshow to a file, and it played fine. So it just seems to be in the preview that it's bad. I wonder if there is a reason for this or if it's a known issue?
Any other suggestions would be appreciated, but for now since I can output the file with no problem, then I'll just continue on.