NetMeeting through Linux firewall

I'm running a RH6.1 firewall/IP masquerader between a US West DSL connection
and a small LAN whose nodes run Windows NT and 98. When I run Microsoft Net
meeting on either machine, I can connect to the other party, share files,
and speak, but I can't hear what the other party is speaking to me through
his mike. Since I get exactly the same failure on two machines running
different versions of Windows and with different sound cards, I suspect that
my firewall is not passing the audio in my direction. My firewall is set up
with the very simplest ipchains script at the moment, since I don't want to
get fancy until all my apps work

Is anyone else using NetMeeting through a Linux firewall?

Alan Gore