I just bought a 665 and the sound was pretty crappy on arrival. Garmin will give me an RMA in a few hours and it's going back for repair/replacement.
Seems like if it gets hot, the sound squeals, dies, or does something in between (unusable in any event). Sound will dump to the external speaker, regardless if anything is connected to it. I haven't tried bluetooth connection via headset (I don't own a bluetooth headset just yet).