I am unable to send SquadSay messages

This is an OSX-specific bug that is currently in the issue tracker (#945). There is not currently a known workaround. Efforts will be made to fix this in the future.

I am sometimes unable to send or receive VOIP communication on highly-populated servers

There is an underlying bug in the UT2004 engine where there can only be 64 simultaneous voice-enabled clients (#887). There is no workaround for this and it is believed to be unfixable without recompiling the engine.