How to connect test leads to TrueRTA Oscilloscope on laptop?

I'm new to making measurements of audio equipment. I'm in the process of adding an amp, speakers and a DSP to my Harley and have been enjoying making some measurements. I've got levels set using a hand-held oscilloscope, and have made some measurements with my PC running TrueRTA. I've got a Behringer ECM8000 mic connected to a Behringer USB Mixer as an input for the audio measurements.
The question is: How do I use the oscilloscope function on the RTA? Meaning, how do I connect test leads to the laptop? I'd love to be able to use the laptop oscilloscope vs the hand-held because it has a nice big display. I know there are a bunch of PC-based oscilloscopes out there that come with a USB interface, but is there a generic USB interface that would work with the TrueRTA software? (Yes, I've asked them, but have had no response).
I'd like to be able to go from an audio measurement directly to a oscilloscope voltage measurement so I could see if there's clipping at various frequencies based on the information from the RTA.