To whom it may concern...

I just found out why my microphone wasn't working in the Linux client, and I thought I'd share my fix. This is with Ubuntu Linux 10.04 and Linux client 3.0.0-beta36 (build 12815), both 64 bit.

There's a sound config applet under System/Preferences/Sound. If you go to the Hardware tab you can select a sound device, and under the selection box there's a drop-down menu with the title "Settings for the selected device". In my case the selection was "Analog Stereo Output" which meant the device was only used for output. When I changed it to "Analog Stereo Duplex" my microphone suddenly sprang to life.

Maybe this helps others struggling with the same problem.