Hello everyone,
I recently got some free time to look into this issue and came up with an app to help solve it. I normally do video game development but I decided to put a solution together for others as well.
I came up with an app that essentially mutes any currently playing music but keeps Here Drive or any other Navigation app audible.
Here is a link to it, I called it Nav Voice Only: windowsphone .com/en-us/store/app/nav-voice-only/bc28fb6c-7b6d-4f7e-a72f-1f9bd23b91c5
(Sorry, can't post a link as a new user, remove the extra space in the link after windowsphone)
Please do let me know if you have any questions or suggestions.