Mind if I post a modified version of this that uses TF2's sound I think this one does not really sound like a hit sound?
Here: http://www.mediafire.com/?eb1ds4fuq8lxt6p
It should be compatible with the Quake 3 version but I highly suggest you don't run them both because everytime you hit someone you'll hear just a mess of sound.
You can pretty much do this yourself.
If the sound is mono, you can basically replace the sound in my .zip with yours.
Otherwise, you need audacity, open the sound you want to replace mine with, press the arrow next to the sound's name and select "Split stereo to Mono".
Delete the second track and export as .wav. Done :D
Oh, also, updated the add-on: now the hitsound is not as loud.