My Zumo has a bluetooth connection, works well with bluetooth earpieces. However, even with a full face helmet, there's too much wind noise to hear anything.
Works great in the car though.
Then I am guessing your problem is :
A. You have a crappy headset
B. You have a real noisey helmet
C. You have no windscreen
D. You did not adjust the output volume correctly.
I have a Zumo and it works great with a Scala Rider Teamset Headset. The big advantage to this one is it has speakers for both ears. Even at twice legal speeds I can hear GPS prompts and make calls on my phone with no issues as to hearing or the folks on the other end hearing. (In fact many are shocked to learn I am calling from a motorcycle traveling at highway + speeds)
I do have a fairly quiet full face helmet and....a fairly quiet windscreen.
But all it not lost. If you just need to hear the GPS prompts or listen to music from it's MP3 player and do not need to use it for your cell phone, you can simply pick up a good set of "in the ear" earphones and wire them via an extension cord to your Zumo.
This provides the best quality of sound for listening to music, and if you get the right type of headphones you should have no problems hearing anything even if B and C are part of your original problem. I rarely use my bluetooth headset but do use my Eytomic ER6i earphones:
These are "sound isolating" earphones and double as earplugs when you are not listening to anything. Because they reduce outside noise, you do not have to turn up the volume on your Zumo all the way in order to hear...as it effectively cuts alot of that wind noise. You can find these online for around 70 bucks.