I am looking to solve a similar problem.. I often use google navigation and google play music at the same time while driving; i don't like the music extremely loud, but if i set the music at a reasonable volume, I often cannot hear the navigation voice prompts clearly.
Will the above app let me turn up just the navigation voice prompts without affecting the music that is playing in the background at the same time? I think that both are outputed on the media audio channel, so if the app is just changing the volume at the level of the media channel it probably wont work.
Any ideas / experience is appreciated..