I have found the same with low audio volume through a bluetooth headset (a stereo crash helmet version)
Phone (gs3), headset and guidance volume in waze are all set to maximum and all I get are whispers in my ears even when stationary. impossible to hear when riding my bike at 30. I know something is being said as the music stops (Lowered preamp in PowerAmp to save my hearing whilst I test this).
However, I have found that if a guidance is being given and a notification sounds, (in this case, Voxer) the speech volume is raised to the level of the notification sound until waze has finished its message. After that the sound is quiet as usual unless it coincides with another notification.
One question from me is: what audio stream does waze utilise? Media?
edit: Waze 188.8.131.52 using UK English - Serena. Not tested with Daniel though