Gnomemeeting doesn't work well with alsa +dmix plugin. because it don't use default device. Here is a patch that is a backport from cvs update made by Damien Sandras. With this patch, audio is now ok when you use alsa + dmix.
Created attachment 42422 [details, diff] patch alsa_sound plugin
Created attachment 42423 [details] pwlib-1.6.6-r1.ebuild update of ebuild to include patch
found it's way into cvs (pwlib-1.6.6-r1)
that's we call a backport...