Qtractor doesn’t have a “render” feature for converting the mixed session to audio. Instead, the audio needs to be recorded in real-time. This can be done by any program capable of recording JACK output.
One way to do this is to send the audio output back into Qtractor and record it there (the Qtractor manual describes this process). Instead, I used the command line program jack_capture (which is pre-installed in Ubuntu Studio 12.04).
With JACK started (using QjackCtl), and with the twinkle1 session open in Qtractor, I opened a terminal window and entered the following:
jack_capture --bitdepth 32 --jack-transport twinkle1.wav
I switched to Qtractor and played the session file. jack_capture was synced to the JACK transport and so recorded only what Qtractor played. File twinkle1.wav was created in my home directory. I closed Qtractor and stopped JACK (using QjackCtl).
I edited the twinkle1.wav file with Audacity (also pre-installed in Ubuntu Studio 12.04) to normalize the audio and to trim silence from the beginning and end. For this test I exported the result in Ogg Vorbis format (which required package vorbis-tools to have been previously installed).
Enter man jack_capture in a terminal window for more information about jack_capture.
jack_capture has a GUI front-end program named jack_capture_gui2 which looks nice but isn’t available in Ubuntu Studio. 😦
I closed Qtractor and stopped JACK before starting Audacity because Audacity doesn’t use JACK by default (in its default configuration, Audacity doesn’t play if JACK is running).