Valve has made improvements to communications in Source multiplayer games in a new update now available - restart your Steam client to get it.
Voice communication is improved thanks to the use of a "much clearer codec", and the update also includes the first release "of a completely reworked in game text chat to which now includes a chat history and a filter to only see important chat messages".
In addition, Valve says it's fixed "not being able to use sounds, decals or models if they were downloaded from the server".