HL2ep1sourceenEngineUpdate

sourceエンジンがアップデートしたようです。

Updates to the Episode 1 Source Engine have been released. The updates will be applied automatically when your Steam client is restarted. The specific changes include:

Episode 1 Source Engine

Counter-Strike: Source, Half-Life 2: Episode 1, and Half-Life 2: Deathmatch customers running under Windows Vista can now take advantage of 5.1 and 7.1 speaker configurations
Vistaでサウンド関係が向上したみたいですね。