Not too sure if this will be a help or hindrance but I had more or less the same problem. My setup is a bit different in the way that my head unit runs Android 6.0 and am using the Igo_Pal with Arami's modified skin.
I found the sound issue was nothing to do with Igo, but in the settings of the head unit. Boki_Srb put me on the right tracks.
If you can go into the factory settings, have a look for system volume and radio volume. I found that Igo's sound runs through the system volume, so it was a case of patience and playing with these settings between the radio and system volume to get the balance right between the two. Also, if you have an option in the driving settings to mix or cut radio off, I found the mix works better. The radio volume will drop but Igo will talk through my front right speaker and when finished the radio will come back up.
It maybe worth a try.
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