I think the soundcard stability is too poor to be able to rely on for phase measurements.  Instead, how about a phase meter that works by comparing the phase of a signal on the left channel input with a supplied reference tone on the right channel.  Then, all that anyone need do is construct a stable oscillator and divider to generate, say, a 1kHz, tone.  

I regularly use a phase meter based on the 56002EVM.  The DSP card performs the digitisation, downconversion and decimation, outputting I/Q samples at user selectable data rates from 800 Hz right down to 1.95 Hz (8000 / 4096)  via the COM port to a PC for further analysis.   In my case the EVM sampling clock is stabilised so there is no need for a comparison tone to measure phase.  

A waterfall plot of phase would be a wonderful and easy way to get a few more dB of sensitivity out of slow signalling over power detection methods..  Even if the frequency stability is insufficient to maintain phase over a symbol period, it should still be easy to spot the data transitions by eye.  The only problem then would be resolving the 0/180 degree ambiguity.

 

Andy  G4JNT

 

Alan - in fact I have been playing with a "phase sensitive waterfall display" which Markus DF6NM suggested over half a year ago. Indeed, the result of the FFT calculation carries the amplitude and phase, but it's not easy to interpret the phase values and putting subsequent FFT results together (all I got up to now is a very colorfull display which looks like "moiré effect" on a TV screen. But with a bit of inspiration, there will be more soon... :-)