This is an extremely common problem, not with ESF, but with Half-life in general. basically if you're playing half-life, and you're in a game, and you minimize the window, you lose all sound (not everyone has this, but a LOT of people do). THe only way to get it back is to disconnect and back up all the way to the first menu, at which point sound will be restored. Although the gfx on your menu may get screwed up a lil.