If anyone is interested in this (aparently no one, but anyway

), you can lower the volume of this particular sound, without affecting the other sound effects.
Open
\Assets\Sounds\XML\Audio2DScripts.xml
Search for
AS2D_SELECT_WORKER
Set
iMinVolume and
iMaxVolume to 0 (or a low value).
Enjoy the game without being scared to death every time a worker is selected !