That thing you put in your autoexec.cfg to get all-talk working on the servers

alias +alltalk "+voiceall; +voicerecord"
alias -alltalk "-voicerecord; -voiceall"
bind KEY +alltalk

Replace KEY with the key you want to use for all-talk, preferably a different botton than the one you're already using for team-talk (which is V by default).

If you're on Windows you'll find the autoexec.cfg file here (or create it if it doesn't exists):

%ProgramFiles%\Steam\steamapp\STEAM USERNAME\team fortress 2\tf\cfg\autoexec.cfg

With OS X it's located here:

~/Library/Application Support/Steam/SteamApps/STEAM USERNAME/team fortress 2/tf/cfg/autoexec.cfg