my wife uses earbud,she gets cheeps cause the cords don't hold up very long,at 7$ apiece their almost disposable.

 

I can hear the Zumo through the speakers just fine with the same patch cord setup as you describe. There are two volume controls on the 550. Go to the wrench (dog bone) on the home screen, hit the volume icon and check that the Navigation speaker is at 100%. If not, push and hold the speaker icon to the right of the value box until it maxes at 100%. Also, increase the volume to max using the buttons on the left side of the display screen. Then you should be able to control the volume with the bike's radio control.