@ perazamarcos ,
I guess your car Radio has different volume level settings for Navigation Volume, Media Volume, System Volume etc.
While iGO is playing an announcement (and if music/mp3 is being played in the back ground), iGO changes the media volume to 7% automatically hence after annoucement, you are unable to hear media audio.
I had the same problem, I used a volume locker (find it on google playstore), which locks all the volume and problem is resolved.
Hope this resolves your issue.