Roxio should be able to convert it itself. Something you need to be aware of is all audio in a track must be exact same format ... can't mix ac3 & pcm on same track. You'd need to create a separate slideshow. Make sure you have the same audio conversion settings when adding the music so it converts to same format.

I use roxio Toast on mac osx, but never/rarely use it to create dvd's & the program is somewhat different to the windows version. On studio pro can add multiple audio files to same track, but it has quite advanced options. Roxio is rather basic in comparison. It may or may not have the ability to add more than one audio file to the same track (slideshow.) It's been ages since I attempted to create a slideshow using toast & can't recall if I had music or not.

What you may need to do is combine all the music outside of roxio using an audio editing program. Then save it as wav. Then add it to the slideshow & allow roxio to convert it to ac3.

It might be worth registering at the roxio forum & posting the query there. It may take a while to get answered though ... at least that's how I found it.