If the OP's problem is the same as mine, it's not a volume issue, but a delay issue. The phone pauses the music in order to get the navigation directions out, but it needs to wait a second or so before it starts speaking. What I hear is: music ... music ... music stops ... silence ... the last few words of the directions ... music again. It's like the act of pausing the music overrides the first few words of the directions.
I do not have a cure for this, but I can confirm the problem.
I noticed (when resetting every damn setting of my phone after last night's post-4.3 FDR) that there's a setting (under Accessibility) whereby you can slow down your text to speech. I haven't tried it, but I expect it might result in you hearing more (if not all) of the navigation directions in this circumstance (although slowing the pace would certainly have a downside).