This appears to be another save project and then open project issue.
The issue is shown with the video created with the steps below. The below tasks simply use the slideshow feature to create a slideshow with music for the first five pics and then another slideshow with the same continued music for the next five pics by using the music mark out and mark in points.
1) put the cherry, flower, lily, tulip, windmill in the time line
2) Do the above again
3) select the first 5 pics and make a slideshow
3a) use cell, add one of your mp3 music clips
3b) (music settings) unclick fade out, set mark out at 10:00 seconds
3c) (slideshow settings) use fit photos to music
4) finish creating slideshow
5) select the remaining 5 pics and make a slideshow
5a) use 3D, add the same music clip as above
5b) (music settings) unclick fade in, set mark in at 10:00 seconds
5c) (slideshow settings) use fit photos to music
6) play clip from start in movie mode, slight click at 10:00 seconds but overall great.
7) save project and quit
relaunch PD7 and open saved project
9) play clip from start in movie mode, second clip has no sound
10) position cursor in second slideshow and play movie and you have sound. Anytime the starting play point is in the first slideshow the second slide show is silent.
I've created disc's after step 6 and 10 and the resulting DVD has no sound during the duration of slideshow 2.
Jeff