Have you looked at the options under Edit>Wizards? Soundtrack Maker seems to be limited to the music the app supplies but the alternative "Musical editing adjustment" may well be helpful to you, though it does depend upon a clear "beat" in your audio track, I think.
If you have 1 photo and you want it the length of your audio, then, on the timeline, simply drag the right side out to the right until you get to the end of the audio track. There is another way, but that should be good enough.