A very simple mod I made to silence that annoying "THUMP" sound when selecting a friendly or neutral unit. Version 1.1b removes the hostile mob select "THUMP" sound too.
I have no idea if this will help anyone else, but if it does, good! I made it after a lot of futile searching. For some reason, it's not in (or not working in) the addon "mute annoying wow sounds".
There is no configuration on this mod. Just unzip and copy the folder to the ..\Interface\AddOns directory (per normal) and enable using the addons menu in-game.