Hey I've had this issue for a while! Using Ubuntu 10.04. Didn't have the problem on 11.04, but I dislike that version.
When I plug in my jacks, the sound comes out of both internal speakers and the jack. If I go to System -> Preferences -> Sound and change audio output from Internal Audio Analog Stereo to RV710/730 Analog Stereo (HDMI), then no sound comes out of neither internal speaker and jack!
Please help me, thank you very much!