Thanks to Matt from the Steam forums for this tip.

How to disable sound downloads

Are you tired of downloading sounds (i.e. “Headshot”, “Ultra Kill”, “First Blood”, “Mmmmmmonster Kill”, etc.) from CS:S servers? Well, add this cvar to your autoexec.cfg file, and you will not have to download sounds from the server:
Code:

cl_downloadfilter “nosounds”

How do I create an autoexec.cfg file?

1. Open Notepad (Start button >> Programs >> Accessories >> Notepad)
2. Type the above code in the Notepad file.
3. Click File >> Save. In the “Save as type” box select “All Files.” Then type “autoexec.cfg” (without the quotes) as the “File Name”.
4. Save the file to your cfg folder.

Once you have done this how do you know you have done it correctly?
Type “cl_downloadfilter” in the console; then press enter. You should see the following:

• Code:
  cl_downloadfilter
  “cl_downloadfilter” = “nosounds” ( def. “all” )
  archive
  - Determines which files can be downloaded from the server (all, none, nosounds)

If you see that exact code, then you have the cvar set to not download sounds.