I didn't like the wind noise you'd hear with the GoPro external mic, and this was usually the reason why I'd upload a music soundtrack to the video, so a couple weeks ago, I tried wiring an external mic from under the seat just above the Scrambler Arrow exhaust using a left over boom mic from a defunct Scala Rider G9 headset. I took the bike out for a short ride to see how it would sound. I thought it turned out ok. The video quality...not so much. GoPro was mounted to the front of my Dart screen, and IMHO, vibrated too much, so I ripped the mount off and I plan on giving it another go using the GoPro "chesty" mount. Sound quality though I thought wasn't too bad and was what I was looking for.
http://www.youtube.com/watch?v=3Yu9ov7G0Us&t=79s