This guide is valid this day, tomorrow it could be totally obsolete, you have been warned.
After having tried every which way to use ffmpeg to screencast audio with video or audio only but always without success as far as audio goes, I downloaded jack_capture and bingo! I didn't see any installation instructions so I just did this:
Thence, with jack runing, I enter
and the rosegarden output saves to a jack_capture_##.wav file in my home folder
It should be a simple edit in the rosegarden code to include an “Export to wave file” menu item.
A provisional workaround under KDE is to
Save the following 3 lines as a text file under something like ~/command/jack-cast.txt & make it exetutable #!/bin/bash # saves to ~/jack_capture_##.wav konsole -e /usr/local/bin/jack_capture
Place a “Quick Access” browser on your panel & set it up for directory ~/command using single or double click activation (trigger).
Next time your rosegarden is ready to play something start jack_capture from your panel with a click and hit the rosegarden play button.
When done hit enter into the terminal window opened for jack_capture.
Copy out the generated jack_capture_##.wav file as desired.
To turn it into an mp3 I done did
ffmpeg -i jack_capture_##.wav whatever.mp3
00:25 EST 19-Oct-2012 ©Kalman Feher, use it freely until blue in the face if you like