I'm not 100% on this, haven't been using Waze very long, but the two *sounds* I hear that don't correspond to on-screen prompts are:
a. Waze rerouting, either due to you passing a turn or street it was directing you toward or Waze losing your correct GPS position (Cell phone GPS's aren't very accurate. If you look at GPS points in the map editor you will see user GPS tracks all over the place) which can result in *multiple* reroutes until it fixes your position again, or -
b. Driving over "road candy", i.e. a points reward for meeting certain criteria. *Note, your vehicle icon will change briefly when you "eat" "road candy". It may also do this when "munching" (driving over unverified roads - shown on map as roads with consecutive blue dots.) Your vehicle icon will appear as a "pac-man-like" icon when "munching".
As far as I'm aware, any other audible cues will appear with a prompt on the display (i.e. hazard ahead, etc.)
Hope that helps.
iPhone 4 / iOS6 *I dont' navigate very often, but when I do, I use Apple Maps . . .