What product did you purchase, and who did you purchase it from?
If you bought "MyDVD Studio", you bought a toned down version of MyDVD. ($49.99 @ roxio website)
If you bought "MyDVD Studio Premier", you bought the MyDVD portion of EMC 9. ($69.99 @ roxio website)
If you bought "Easy Media Creator 9", you got the full suite, including VideoWave. ($79.99 @ roxio website)
From the sounds of the price you paid, you either bought the MyDVD Studio Premier, or MyDVD Studio. If this is the case, you unfortunately have no access to VideoWave.
The "full" version you are referring to is probably "Easy Media Creator 9" (EMC 9).
Now, to address your actual problem.
I did some testing to double check this, and if you used the slideshow assistant to add all of your photos, the minimum time that MyDVD allows you to have your photos displayed for is 2.00 seconds.
With 18 minutes of audio, thats 590 pictures, so that might be where your problem is fitting in.
Add new Movie
If you go to add new movie, and then select "edit movie", you wind up in a Videowave-esque screen. I'm not sure what will happen if you do this in your MyDVD Studio [Premier?].
From here you can much more easily customize how your photos are displayed, but it will take much more time to do.
I was able to highlite a few photos and add them with the "Add new Movie" button from the MyDVD main menu, and it just threw them in a movie with no transitions, and the default duration. You can just edit the length of the photos from here.
And now for the last part
Because this program is MyDVD, the standard for DVDs is a certain number of frames per second. If your settings were 30 frames per second, doing the math thats 0.033 seconds per frame, which would explain the amount that it is letting you change the duration by. (1.07-1.03 = 0.04, but if you increase it from 1.07, it jumps to 1.10 which = 0.03. So the program rounds.)
Do the "Add New Movie" method, and just make the last slide 2 or 3 seconds longer than it usually is to make up for the extra audio time.