Because the PC sound card is used, only AC signals in the ~20Hz-20kHz range are measurable directly. Their amplitude must be at or below line level.
However, to monitor (slowly varying) DC you could use a voltage-to-frequency converter to drive the sound card.
I was going ask again about connecting 9VDC to a sound (1V) input. Then the import of alec_t's post hit me! I'll look into that interface a Googlic-ious search
make your own probe with 10:1 divider R ratio using an audio extension cable and stereo Jack.
Measure input resistance on AUX channel to choose series R. Some are DC connected.
If not , shunt with suitable Rp and add series R's for 10:1. Calibrate with 5V
e.g. 10K shunt 90k series for both channels. up to 100k, 900k
Change Windows Recording source to AUX enabled and Mic OFF