Sound modding in Black Mesa is surprisingly easy compared to something like Left 4 Dead 2, all you need is audio software like Audacity (completely free) and GCFScape to extract the sound files from the vpks.

- Open bms_sounds_misc_dir.vpk (found in Black Mesa\bms) using GCFScape and extract the root folder to wherever (Desktop for example)
- Dig through the folders until you find the gib sounds
- Delete every other folder except for the ones leading directly from the root folder to the gib sounds
- Open the gib sounds in Audacity
- Go to "File/Import/Audio..." and import your replacement sound
- Delete the track with the original sound
- Go to File/Export/Export as WAV, leave everything as is and overwrite the original file
- Once you've done this for all the gib sounds, take your root folder (rename it to whatever you like) and drag it onto Black Mesa\bin\vpk.exe
- A .vpk file with the folder name will appear in the same location as the folder and that's your mod, just put it in Black Mesa\bms\custom (make the folder if you don't have one)

Here are the download sites for GCFScape and Audacity:
https://nemstools.github.io/pages/GCFScape-Download.html
https://www.audacityteam.org

I checked myself and if you're after the hl2 impact sounds they're found in Black Mesa\hl2\hl2_sounds_misc_dir.vpk "root\sound\physics\body"