TomTom in the iPhone is the best GPS available anywhere. Better even than the TomTom standalone devices. The combination of IQ Routes plus live traffic puts it above the rest. Just today it had me get off the highway litterally just before traffic started, getting me there in half the time as my friend who stayed on the highway. Despite having to route around traffic, it's original arrival time estimate was accurate
to the minute. It's $36.99 right now, and has exactly what you're looking for.
That being said, Navigon has the audio setting as well. Make sure you're fully updated and go to Settings -> Sound -> Volume. The volume here controls the voice volume and is independent from music volume. I just tested this with both apps, and it worked as expected.
As a bonus tip, try out
Navigon Now, an easy way to copy->paste addresses into Navigon. If you end up going for TomTom,
Paste 'n Go does the same thing.
Good luck!