Equipment required to measure audio line-out voltage peak-to-peak?
If all you want is to find the peak value, one can potentially connect a diode and capacitor at the output of the tape recorder, and measure the DC voltage (something like a sample and hold).



simulate this circuit &ndash; Schematic created using CircuitLab
Also, an arduino can be set up to repeatedly sample the voltage and display the maximum.
Also, you can calibrate your multimeter with a square wave signal (because you have digital signal on your particular cassette) of known amplitude, then multiply tape recorder output with that multiplier.
Also, you can feed in 50-60 Hz pure sine wave of known amplitude into your tape recorder and measure the AC output. Most multimeters are fairly accurate at least in the 50-60 Hz range, if the sine wave is not distorted (even more true if multimeter is legally certified).