Steam's voice chat system now leverages the SILK audio codec, developed and used by Skype, makers of the world's most popular voice communication service. The SILK codec provides a significant quality improvement over Steam's previous voice technology, at the cost of some increase in bandwidth usage. Steam Voice used to require 15 kbps of bandwidth, whereas SILK is a dynamic bit rate protocol which varies in its use of bandwidth between 8 and 30 kbps, depending on the range of data in the voice signal and current network conditions.
As of today's Steam client update, voice chat using SILK is available to all users of Steam. To start using Steam chat with SILK, simply click the 'Start Voice Chat' button within a friend or group chat on Steam. You can access chat from both the friends list at the desktop, or while in game using Steam’s in-game overlay. You'll find voice chat connectivity and reliability have also been improved with this release.
Steam chat with SILK is now also automatically available for all games that take advantage of the Steamworks Voice API. Valve's own Portal 2, set to release in mid-April, uses this newly updated system to enable voice chat in its cooperative gameplay mode.