Hi Fadboy, Yes music can come out of the speakers, though with the handlebar mount you do need a small speaker extension lead.
There seems to be two ways of connecting the bike speakers to the Garmin 660.
(1) Via the socket on the side of the 660.
(2) Via the audio out fly lead that comes attached to the Garmin (part of the power lead). The dealer who fitted mine actually located this connector under the seat, though still accessible. At first I thought it would have been better to have it by the speakers at the front but after taking the screen off I soon realised that this would not be a practical position, so under the seat is probably the best position. Also when it's under the seat it would be very easy to attach an ear piece in an emergency, if the Bluetooth helmet headset failed.
Hope this helps
Cliff