Ahem, I forgot to include a link to my "mod" in the opening post:
http://www.wowinterface.com/download...-EarPlugs.html
You can find the zip file download there.
If you want to create your own you need to extract the relevant files which are stored in the mpq archive files using some form of mpq archive tool e.g.
mpqviewer. Note the directory structure of the archive. After saving the files from the archive you can play about with them as you wish, I used Audacity to simply lower the volume. If you then recreate the directory structure in the Data folder (just the relevant bit - there's no need to do the entire thing) and place your modified files in the appropriate place they'll take precedence over the archive files.