Hopefully lei will enable this by default or add a GUI menu in the next patch/version. (HINT HINT)
cl_voip and sv_voip is enabled by default (almost all servers support it unless they restrict network rate). s_useOpenAL is enabled too but most Windows users don't have openAL and Linux users might be bound to the wrong version.
People need to bind a key but you can already do that in options somewhere.
The next version will restrict voip teamchat by default in teamgames, making it more useful for cooperation.
There are 3 things that stops voip from working:
1. openAL is needed for recording sound (ioquake3 might allow the default SDL sound later, but recording is apparently a problem). OpenArena does no longer ship with an openal.dll
2. Rate must be set to 25000 and the server must allow a rate of 25000. Might be a too high requirement a ISDN or Wireless connection should be enough.
3. Legacy binaries does not support voip.