Updated Article - Nov 2013

In ProSelect v2013r1 and newer, all slideshows are controlled by a Slideshow Style which controls which music group to play, slide and transition times, background and border colors or images as well as other effects and options including syncronizing your slideshows with your music.

Syncing the slideshow to your music will adjust the slideshow slide and transition times within your specified allowable range so that the slideshow and music will end at the same time.
Music syncing will occur over multiple music tracks if required. If syncing is not possible, your “preferred” slide & transition times (as set in the current Slideshow Style) will be used and the music will fade out at the end of the slideshow.

You will find more information on "Slideshow Music & Timing" and "Exporting a Slideshow Movie" in the ProSelect Reference Manual (quick access under the Help menu in ProSelect).

If using ProSelect v2012r2 and earlier (original article)

Fading the sound at the end of the QT slideshow is not possible, however it's not usually needed since ProSelect's QuickTime slideshow export function is designed to allow you to easily adjust the slideshow timings so it finishes at the same time as the music.

With the Add Music box checked (and some music selected in the either the Preferences--> Slideshow tab or the Album Menu->Slideshow music), adjust the slide and transition time sliders until the blue bar (representing the length of the slideshow) ends at the marker showing the end of the music track. It's usually better to have the slideshow end 2-4 seconds before the music for a nice effect.

You can see the actual ending times in minutes and seconds displayed.

Audio Level Control

At this stage it's not possible for us to add any audio level control to the movie. If you have a sound editing program you could edit the music down yourself to a shorter time and add the fade or choose a shorter music track.

As far as Sound editing software goes, one option for Mac is Amadeus from this link www.hairersoft.com/Amadeus.html.