bug # 292061 missing media-sound/jack-audio-connection-kit dependency. Perfences dialog does not working without this package. Reproducible: Always
This dependency is already there when the jack USE flag is selected... is it still needed even without the jack USE flag? Can you try?
I've just installed gtk-recordmydesktop and I encountered the same issue. I can confirm that even without "jack" use the "Advanced" config option won't work without installation of media-sound/jack-audio-connection-kit.
This bug is a dupe of bug 311689 and bug 321313. There is a proposed patch on those bugs that fix this issue. Finally, there is a patch in bug 283489 that (IMHO) should have been backported into the portage version of recordmydesktop (else, JACK support does not work).
Fixed with bug #321313.