Messenger voice doesn't work

[richard]

An update.

I have three machines using Windows XP home edition.
They are all new. One is a Sony Laptop and the other two
are custom built desktops. Both the custom builds are
made with high quality components like Asus and Intel
motherboards and fast processors. They are configured
well and work great in general. All of them share a
linksys Four port cable router. It is Upnp enabled.
They are sharing a cable connection. Each machine has a
messenger account. On the laptop and on one of the
desktops I can use the full functionality of Windows
messenger including voice and video-it works perfectly.
They can call each other with voice and video and can
call people outside the network too with voice and
video. I just got done with a long video conference in
fact. One desktop, however, can only receive sound and
video, but does not send out any. It passes the audio
wizard and sound recorder records sound. When in
messenger it connects fine to other machines and it
appears to work but no sound comes out the other machine
even though the mic appears to be working and its levels
go up and down when I talk. It doesn't matter whether I
call a machine behind my network or outside of it. I
think I know how to configure my machine and router
properly as two of my three machines have no problem
using all the features of messenger. I thought perhaps
it was related to the onboard sound card so I disabled it
and installed a Soundblaster. Even with the soundblaster
card it did the same thing. Can only receive sound.
Anybody got any ideas?

Thanks.