You need to use an app that supports that.
Alternatives to iPhoto's slideshow include:
iMovie, on every Mac sold.
Others, in order of price: PhotoPresenter $29
FotoMagico $29 (Home version) ($149 Pro version, which includes PhotoPresenter)
Final Cut Pro X $299
It's difficult to compare these apps. They have differences in capability - some are driven off templates. some aren't. Some have a wider variety of transitions. Others will have excellent audio controls. It's worth checking them out to see what meets your needs. However, there is no doubt that Final Cut Pro X is the most capable app of them all. You get what you pay for.