Hi.

Just to be clear, you want the music to end at the same time as the slideshow?

If so, the "easiest" way is to simply cut the audio track at the end by, using the Timeline not the Storyboard mode, placing the cursor at the point where you want the music to stop and Pressing "T". This will split your audio track - you will now have two audio objects on the timeline - simply highlight the one you do not want and hit the Delete key.

However, your music will now just suddenly "stop". To avoid this you can use the on-clip volume control - it looks like a white dot right at the end of your audio track - and drag it backwards so the end of your track now looks like this image:

I have added the red box to show the point to which I dragged the white dot; the gradually shrinking light blue area from there to the end shows the audio track is "fading out" (to nothing by the end).

HTH

Jeff