An official statement has been made together with the release of the beta saying "This beta is addressing some regressions with the recent game patch". Also "Note that silent/Russian walking was an exploit and has been fix". If you want to try it out "You can access the beta by going to the Settings dialog in Steam, go to the Account tab and hit the "Change ..." button under the Beta section. Then select the "Counter-Strike 1.6 Beta" from the list".

This is the official change:

* Fixed all sound related glitches with the Russian walk / silent running fix (including standing still sound, repeat sound, and always default sound)