I haven't seen any hitsound of a piano so I figured I'd make one and share it and the result is pretty nice.
1. Download This .wav file
2. Put the "hitsound.wav" file in this directory: C:\Program Files (x86)\Steam\steamapps\common\Team Fortress 2\tf\custom\my_custom_stuff\sound\ui
3. I use it with these recommended settings:
put them in: C:\Program Files (x86)\Steam\steamapps\common\Team Fortress 2\tf\cfg - autoexec.cfg
4. In game, in the advanced settings check the box that says:"Play a Hitsound Everytime You Injure an Enemy"
5. In the same place, in the section: "Hit Sound" put it on: "Default".
You can also download it from here: https://huds.tf/forum/showthread.php?tid=1951