ProSelect controls the standard sound output device volume level to provide smooth music fades at the end of the slideshow. 

However, it cannot control the sound volume if you use other audio output methods such as Optical out or USB-audio convertors. Therefore you should always use the standard audio jack for connecting your external speakers to your computer.

Update August 2012

In July/August 2012, we had a few reports about the slideshow music not fading (it was simply continuing) and many found that restarting their computer (closing applications and turning off the computer and restarting)  resolved the issue.  It was reported across old and the current ProSelect versions (Mac and Windows), so it points to something on the computer that had changed recently, possilby with an update  (eg  Quicktime software, the Operating System etc).