I use JACK + Ardour for music recording and mixing. For listening, I typically just use a laptop's headphone jack. This can lead to strange and confusing behavior. Well, no more: at least on my laptop, this all can be avoided by connecting Ardour's master out to system:playback_3 and system:playback_4 in addition to system:playback_1 and system:playback_2. This way, the master out goes to all the software channels, and somewhere, Ubuntu decides whether the headphones are plugged in and only sends the output to the correct place.
