That project is quite limited, doesn't provide much protection to the soundcard or the PC it's connected to. Depending on what you plan to meassure might be enough, let's say you want to test small audio projects running on 9V batteries, that's ok. Mains connected SMPS not ok.
I don't know what project is good for this, it will always be a very limited tool, but I'd try to get a buffered amp, high impedance (1M would be optimal as standard 10X proves would work with it and they are cheap) and input should be protected somehow, some series resistance and reasonable clamping would be a first step. With a soundcard you will always have AC coupled input so might be a good idea to make the input of your circuit also AC coupled as you won't know if you are clipping due to DC offset in the input or the signal is just like that.
Also you might want to have switchable gain, 0.1, 0.2, 0.5, 1, 2 ,5 and 10 would be a nice start.
For the signal generator you might also want to have a buffer, standard is 50? output but sometimes for audio is good to have a low impedance output (an opamp, maybe driving some extra transistors, directly to the output) and then your switchable 50? resistor. Also here would be good to have a few selectable
This config would be a good starting point I guess, just 4 opamps, a few zenners to provide clamping and some passives, switches and connectors. At all time using it (and before getting into the project) you should have in mind that the thing would only work from 20HZ-ish (maybe less depending on the soundcard) to 20kHz-ish depending on the sampling frequency. I've done 90kHz-ish measurements with my sound card, but mostly testing analog audio stuff which doesn't need any special connection to the soundcard. I don't remember using it as an osciloscope for other than that, and even then more than an osciloscope an audio test setup. I did use some low impedance output buffers when testing some stuff.
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