stepbill, I also have the Zumo 550...and mine is hardwired into the bike. You can wire the GPS to listen to the voice directions so that it can be heard through helmet speakers...done via blue-tooth...or wired via a GPS/Intercom unit from Sierra Electonics. But I do not know of a way to route the voice directions through the external speakers of the bike.
For hardwiring the GPS, it is so simple, it is NOT funny, so don't laugh
Under the LEFT side pocket (left side of the fairing, the pocket that has the spring-loaded lid on it), once you remove that entire pocket, there is a 12v source there, inside the clear rubber boot. The connector for that power cource is a RED connector. In my case, I used the Y connector, or splitter, so I could connect to that RED power source connector...split the power...and then feed both my GPS and my Valentine One radar detector. You can use the power cord from Garmin, wire it into a female connector that will snap into the RED male connector that the bike comes with under that left pocket, and then route the Garmin power wire up to your GPS.
This is were I have mine located, and wired to: (things have been changed since the lightning strike, but you get the idea)
Attachment 4043