Prior until now I've been worried about having to have music at a high volume to overcome wind noise or just plain having too much noise in the helmet - I hate wind noise and also didn't want to increase wind noise by removing ear-plugs and adding speakers of ear phones.
In the end what I've done is hack up a helmet speaker system that I got with a walkie-talkie years ago (in a hopeless attempt to have bike to bike comms) and mount the speakers in my helmet. This then plugs into the Zumo for the music source. The way that the speakers have been placed also allows me to still wear my custom ear plugs too.
So does it work ok? Well, yes - good enough for me anyway. With ear plugs in and the Zumo volume on 100% I can hear the music just fine (at pretty much any speed) and it is not too loud. Naturally with the ear plugs in the wind noise is kept at bay too. Trying the system the other day without ear plugs had me running the volume at 60% but the wind noise was interfering with my listening pleasure. Another thing that I'm happy with is the speaker placement - I was concerned that they might rub or annoy my ears but even with the ear plugs in they do not touch anyway and the helmet remains comfortable.
Distracting? Well, I don't think so. The system is not loud and although I did find myself singing occasionally (had to shut up as I rode through Marton), I was able to navigate the bike through some wet and windy back roads without ending up in a fence. The good thing about having the music coming from the Zumo rather than an ipod in my pocket is that I can easily control the volume of turn the music off if necessary.
Now, to get a bigger SD card for the Zumo...