There are already few discussions about this and similar problems.

On Android different sound channels have different priority.
Try with changing navigation_audio_stream value (audio channel) in sys.txt:
Code:
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This value ("4") is almost always muting all other audio sources.
Probably if you set it to "3" it will be muted while talking on phone, but navigation messages will not mute music player, and will be "mixed" with music.
Try to play with this values and settings in your device.