Better late than never, and this might help someone else searching the forums. This should also help with Google+ Huddle which uses the same plugin.
Open a terminal session and access the Google Talk plugin folder in your user profile:
Make a backup of your current config file:
Open the config file with your preferred editor:
cp options options.bak
Change the following line:
gedit options #alternatively nano options
So that it reads:
Save and close your editor, then log out and back in or restart your computer.
The Google Talk plugin should no longer adjust your microphone settings automatically, allowing you to set it as preferred using alsamixer or your preferred audio adjusting tool.