im with HSU and sonic boyster on this one, ESF is way to fast paced for voice chat to work. And in the modes where voice chat would have a legitimat use, it would be pointless since the focus of ESF is not team work but fighting. Unlike other HL mod like CS, DOD and NS there are no hosteges to rescue, flags to capture or enemy bases to destroy. Your objectives are just kill and not get killed, even in teamplay your are not rewarded for helping your teamates; so there is very little reasons to add this to ESF, since teamplay is not the focus. Sure you can argue that Capture the Dragon Ball mode could use this but even then that part of the mod is still months if not years from being complete, and thats IF the dev team still wants to do a CTDB. Sure you can mute the annoying people but you acctually have to do something to mute them, And how many people would you have to mute on a server just to make your ears stop hurting?
And besides if you want to chat -- log on to MIRC not ESF, its a game not a chat room.