It won't go through the head set or the bike's speakers.
I've had mine glitch when I was running the radio's Aux (660's Media Player)/CB/cellphone bluetooth all at the same time... I have the navigation prompts muted. I actually received a call on the phone, the GPS prompted me that a call was incoming, but when I went to answer it I got nothing.
After that glitch, I noticed even the media player's audio stopped coming through the headset. My solution was to pull off the road, attempt to call back the phone that was incoming only to get nothing... lucky for me it was just an automatted message from my doc.
After not being able to get anything through the system, I did one of the first things everyone should try at least once... I turned the GPS off and then on again and everything went back to normal.
Can't explain the glitches, but everything reset after the reboot and hasn't resurfaced... at least not yet.
I know it's not much, but you're not alone... there are a couple other threads on here dealing with the 660's noticed problems.