How can I calibrate my sound card based oscilloscope?
The only way to truly calibrate an oscilloscope is with a calibrated function generator. A quick internet search will demonstrate that these things are usually not cheep, of course.

...but you don't need perfect, just "reasonable"...

The best way to do this, then, is to:


Build a variable frequency square-wave generator (CMOS 555 FTW)
Use your DMM to measure the output peak voltage with freq set to ~DC.
Bump the frequency to ~1k and adjust the scope.


Provided the square-wave gen's impedance is fairly consistent over all frequencies used, the peak voltage that appears on the scope should reflect the DC voltage fairly accurately. Knowing this we know that it's just a matter of getting the scope to agree with the DMM.

If you want more confidence, repeat the process with your gen powered from more voltages, then just compare the scope output to what you expected.

Lastly, I would not rely on the AC function of any DMM to be useful for something like this. Those things are usually specified for specifically 50 or 60Hz, and will read inaccurate for other frequencies. And, in case you're thinking you've got a workaround for that, you should know that your sound card will (read: should) take to 50-60Hz like an NSA vending machine to a crinkled Won.