hello Joan,
you have a couple options. I am assuming you are using iPhoto to create your slideshows (and iDVD to burn them to disc). In iPhoto, you can only add 1 music track per slideshow, but you can have multiple slideshows on one DVD.
So you cannot have a music change during the same slideshow. However, you CAN have multiple music tracks in one slideshow if you use iMovie instead of iPhoto to create it.
Multiple slideshows using iPhoto/iDVD:
Create all the slideshows you want in iPhoto, applying 1 different music track to each.
Select the first slideshow, then go to Share > Send to iDVD.
Once your iDVD project has been created with the first slideshow, go back to iPhoto (with iDVD still open) and select the second Slideshow and go to Share > Send To iDVD again.
Rather than creating a whole new iDVD Project, this will add that slideshow after the previous one. Keep going back to iPhoto to add more Slideshows if you wish.
When the DVD is watched, and the user finishes watching the first slideshow, they will return to the main menu where they can select the second, and so on.
Create 1 slideshow with multiple music tracks using iMovie:
To do this, you will need to use iMovie. You can make exactly the same slideshows in iMovie that you can in iPhoto, but it takes a bit more effort, because (I don't think) it has the same auto-features, such as "fit slideshow to music" and automatic cross-dissolve between photos.
Anyway, create your slideshow in iMovie. Once you are done placing the photos, switch to the TImeline view if not already (the button with the Clock icon on it at the lower-left).
Click the "Audio" button to the right and choose the first track from iTunes, dragging it in the timeline view and dropping it in. Go back to the iTunes music list and drag the second song into the timeline, dropping it after the first.
Actually, it may be better to drop your audio files first, so that you know how long you need the slideshow to go for, but whatever.
I hope that helps you. I was pretty lapse on the in-between steps, so if you need more direction post back here :/