I have been trying a few set ups with the Zumo 550....
Using a Jabra BT320S Bluetooth stereo headset (which you can add your own speakers/headset) paired with the Zumo....Good sound when using the headset...but headset not really suitable for M/C use...so should be good with custom monitors....I do not think the music from Zumo Bluetooth is in stereo?....So now added a Jabra A120S Bluetooth music adapter... this plugs into the Garmin bike cradles 3.5mm audio jack. Which I think is a stereo output.... this adapter (smaller than a pp9 battery) then transmits stereo over Bluetooth to the Jabra headset....With the headset set to around half volume and the Zumo set to around 50% on music and directions it is more then loud enough with room to spare..
I did try the Jabra headset going through a cheap pair of helmet speakers in my Arai Tour X. But I could not hear very well at full volume on both units and my Remus

I also tried these helmet speakers with a direct feed (hard wired) from the Zumo...this was marginally o.k. but defeats the object of a wire free setup.
Must be an impedance thing????
So to sum up Jabra BT320S (Bluetooth stereo headset) paired to Jabra A120s (Bluetooth stereo adapter) which is plugged into the Zumo bike cradle. Then your phone can be paired to the Zumo...which you can hear through the Jabra headset......BUT no Microphone to speak into (answering the phone)

....have not worked that bit out yet......
But I never use the phone whilst riding/driving so its ok for me...
Now to save up for some custom ear monitors for the old earbowls....
Still playing
Tazwick