Not Logged In
What is a ding-a-ling?
It's (by default) a bell noise that you hear when you injure an enemy.
How do I turn on the ding-a-ling?
In the main menu, click on "Adv. Options" and look under "Combat Options". You should see a "Play a hit sound everytime you injure an enemy." Tick that box and you're done.
How do I install a custom ding-a-ling?
To use a custom hitsound, download one of the sounds from here and put it in your tf/sounds/ui folder.
If the folders don't exist, make them.

Note: Make sure the sound file is named hitsound.wav
How do I uninstall it?
Simple, you delete the sound file and it should go back to normal. (tf/sounds/ui/hitsound.wav)
Is it possible to have different hit sounds per character?
I'm afraid it is not. You used to be able to but due to an update for TF2 this is no longer possible.
Are you another full-fledged website that caters to an insignificant purpose dealing with TF2?
Yes we are, and it serves the purpose of preventing millions of different threads being made about their ding-a-lings and helps people find one they want easily.