Excellent question - I had the same one! Good answer too although it's not very smart of them to make it requiring that a sound source be chosen in order for the gps to talk.
My problem is making it so that the gps voice is loud enough compared to the radio music volume. When I have music at a good volume, the gps voice is very soft. I've been into the settings menus and set gps volume to maximum both in the gps and in the music settings. I also set the radio source volume to the lowest (gps at 12, radio at 0) and they're still ridiculously unbalanced. The Jensen documentation says that if you adjust gps volume while it's speaking, after done, it will go back to radio at it's previous volume. This does not happen. If I turn up volume while gps is speaking, radio comes back on very loud. I think I've been through everything in the settings - anyone else have any ideas?