If you have purchase music from an on-line music store such as iTunes, then there is a good chance that this music is protected and can only be played once it has been registered on a particular computer. 

Normally this is automatically done for the computer which you used to download the music. However, if you then copy the music to another computer (or try to play it across a network from another computer) then make sure that you register it for that machine BEFORE you attempt to play it as part of a ProSelect slideshow.  Note: Itunes only allows you to register protected music to be played on a limited number machines.

You can check if any music needs to be registered by attempting to play it from outside of ProSelect first. If you do not do this then you may need to start the slideshow again after registering the music but, more likely, you will need to close and re-open ProSelect before the music will play correctly.

Another way to check if an iTunes music track is protected is to open iTunes and look in the Kind column for that music track. Protected music will appear as "Protected AAC audio file" or something similar.

Using in QuickTime Slideshow Movies

If you are creating a QuickTime Slideshow movie to give to your clients then make sure that you do NOT use any protected music in the movie. If you do then, while it may appear to play fine on your computer (provided the music is registered for your computer), it won't play on the client's computer for the above reasons.