I'm using Pulseaudio but sometimes I also start Jack and it's like Pulseaudio is left in a suspended state without terminating, this is on Debian SID.
With gmb set to audio autodetection I can't get it to play thru Jack, but selecting Jack output it works. Maybe autodetection is trying Pulseaudio since the process is still running.
Sorry, the auto-detection is a feature of gstreamer (http://gstreamer.freedesktop.org/data/doc/gstreamer/head/gst-plugins-good-plugins/html/gst-plugins-good-plugins-autoaudiosink.html), maybe they would accept a bug about it, I don't know.
Ok, thank you for the hint, I'll do a bit of research.