Comment 1 for bug 131879

are you using esound, pulseaudio or something similar which allows to mix multiple audio streams, or you try it using alsa or oss directly?
you can check this with gnome-sound-properties

i just test this under gutsy, and right now I'm in a conference at ekiga and the same time listening podcast in rhythmbox, works as expected here.

ekiga 2.0.9-0ubuntu3
rhythmbox 0.11.2-0ubuntu2
esound 0.2.38-0ubuntu3
gstreamer0.10-esd 0.10.6-0ubuntu1
gstreamer0.10-alsa 0.10.14-1ubuntu3