Zumo 550 ist going to set the world on fire with it's bluetooth.
They used a mono bluetooth chip which snaps crackles and pops when given anything stero to work with.
Nav instructions are ok, there mono.
Best solution i found is
http://www.ukgser.com/forums/showthread.php/480886-Bluetooth-MP3-in-stereo
Audio music from the garmin goes via external bluetooth dongle
Spoken instructions (nav) are sent via the zumo internal bluetooth
I know you dont want bluetooth, but your going to have to fit leads and speakers in your helmet anyway.
A bluetooth module takes up very litle room
And to echo Russ996Duke - i wear plugs too, you need to wind the volume up
Bluetooth means no leads plugs to get wet & corrode , foul and get ripped off etc and it goes with you on any bike
And you can make - takes calls on the go - saves stopping
If you are that determined to get just spoken nav. Pretty sure you need a 3.5 speaker male to either a 3.5 male or female (depending what the input on the autocom is )
Then just plug it into the garmin mount lhs and feed it round to your autocom box input i think 3mtr or 4.5 mtr would be ok -
DONT QUOTE ME on the length - you need to measure - you prolly wont get that long off the shelf at a reasonable price so
ebay will be your best bet