Unfortunately Jack, that's normal for sound on bikes. To get decent sound you have to invest in decent purpose designed microphones as I found out today.
Do you know what encoding your avi files are? I suspect they will be mpeg layer 4. (note a file doesn't have to have an mpg or mpeg extension to be coded mpeg). If that's the case then there aren't many video editors that will work happily with mpeg. Your best bet is to convert the files (I use Movavi) to true uncompressed avi to edit and then convert back to divx with an avi or wmv extension for uploading to sharing sites.
I use Adobe Premiere to edit my videos although any video editor will work well with uncompressed avi.
Hope that helps, but if it's all gobbledegook let me know and we can get together sometime and I'll happily show you.
Regards,
