I hate to see a first post question go unanswered. I use Showit Web.
Showit Products - Showit Effects, Showit Web
I can't answer your question directly on how exact the music and slides can be synced as everything I do is much less demanding. I bought the $99. version 2 years ago and within a day sprang for the pro ver. for another $99. I had slideshows running within minutes.
I like it's simplicity on my end and customers have been very happy with the results. There is a fairly active blog and forum on the web site but I have not been on either.
The other option is jAlbum
Jalbum - Photo album your way - Free Jalbum web album software - Share albums with friends I am trying to set some time aside in Aug. to learn everything needed to make a custom show with pay site and music with jAlbum. The beauty of this is that there is a very extensive forum and hundreds of people adding 'skins' since it is freeware. I just need to take the code from half a dozen different ones and paste it together. Seems when I do that, something goes wrong everytime.
I'd appreciate hearing anything you run across that incorporates Paypal.