who installs after market navagaiton systems in an h3 2007

at sewell hummer in dallas texas, I had navagation system installed.  Now my onstar volume is so low that it cannot be heard and the voice on the navagation is also extremely low.  After it was installed they now tell me that nothing can be done and mine was the first one they had ever done.  My question is can this be fixed??  Why would they tell me of this option if it had not been checked out before hand.

Thank you for your help and response to this matter

Joe Brooks