I have the same issue, I finally took delivery of our 2014 JKR we ordered this Friday night.
Used the Nav. for the first time last night. I can not hear the voice prompts on the 730 Nav. WAY Too much base to understand whats being said. So far I hate it!
I did find the voice volume adjust under menu, but couldn't find a way to adjust the voice base or treble. As of this moment, We're Very unhappy I chose to build it with the 730N