Caveat
There are lots of soundcard based PSK31 packages to choose from in the Ubuntu sources.
I have only looked at each one very briefly so I may well have missed some important features and options.
Most of these programs offer additional modes to BPSK31 which is all I've checked so far.
Some programs failed to run immediately and I have not looked at those at all - there
could well be some superb programs I've missed out for want of a manual install.
Initial findings
Ran immediately and the window at 715 x 450 pixels fits well in the screen space available.
I prefer waterfall type displays to the spectrun scope shown here.
I found the visible bandwith of less than 1KHz rather restricted.
Decoded BPSK31 well
Future
This program deserves a more in depth look if only because of the ideal screen size.