I think the PC-scope would be adequate, aside from the fact that the sound system is apparently filtered to cut off at 20 Hz and, therefore, won't allow DC components below 10 Hz to slip through. For audio work this is entirely understandable. But for the filter, I'd guess that this might work with some sort of subtractable carrier, but with that filter ... <sigh> ... it's not likely.

I guess the trick will be to sample with a vehicle such as that suggested by Michael, though it's rather costly, and synthesize a trigger based on the transition, positive or negative, beyond a particular set-point. I'm persuaded that that SiLabs 'F12x eval kit might work, too, though it won't use the USB.

The heavy lifting, i.e. the 'scope display and its features, can be handled with the pc-scope software, but the sample stream has to be converted to a .wav file to be introduced. I'm afraid I haven't sufficient motivation at the moment to write a tool to produce those .wav files.

RE