I'm using Soundcard Oscilloscope software that presents as below
You'll notice that the display has a grid, and in the example is set for 100mV/div. Since it's using the sound card integral to my motherboard, there is no calibration of this display or the line in socket, so the divisions could actually be anything.
I have a digital multimeter as well that can measure AC style voltages in the 10 - 400 Hz range. How can I calibrate it, at least roughly? My only thought was to build a sine wave oscillator to produce 100 Hz @ 1 V RMS. That way I can confirm the amplitude with my multimeter, feed it though line in to the software and then adjust for the reading.
Is this likely to work, or is there some other way of calibrating PC based soft oscilloscopes?
Following some comments, I tested my multi meter against a range of sine wave frequencies. These were line outs generated via Audacity software, with the TONE command set to a volume of 0.8. The red dot is @ 50Hz, so this should be ideal for the meter and forms a baseline voltage reading. The response seems reasonable over 10Hz - 10KHz. Even better across 50Hz - 1KHz.