Karl Beckman wrote:
There is an issue of useable bandwidth. Sound cards generally don't respond
to much above 20 kHz. Do you propose to convert the RF signals down into
the audio baseband without destroying the information which you are trying
to analyze?

That is no problem when you want to analyze narrowband signals, isn't it?

With a stereo soundcard sampling at 44.1 kHz and I/Q mixing you can
handle a 40 kHz wide portion of RF spectrum. Enough for most modes
used by radio amateurs.

And you could always get a better soundcard (sampling 4 times higher).