While I do believe there is a much simpler way of removing sounds from the game without having to go through each audio file, including audio files from hl2 source, I was unable to find any resources that would assist me (at least none that seemed straightforward to me).
Wouldn't it be possible to have a custom scripts/game_sounds_manifest.txt with the following content?
game_sounds_manifest
{
}
(so it doesn't preload/precache' any sound definitions file which sets up any ingame audio files)
---
I don't have any demos with people talking in it to test, but in theory creating the manifest file above and putting it in /custom/nosounds/scripts/ could work.
This 'trick' to get rid of sounds does not work on non-community servers with sv_pure > 0, because the game_sounds .txt files are marked as trusted_source.