[Al Skierkiewicz]

Seeing as this is a software only device, one must assume that the bandwidth is only what the sound card is capable of handling. Most sound cards are designed to handle an A/D conversion for CD quality audio which means, best case bandwidth of 22kHz with acceptable distortion of an audio signal. Add to that the aliasing of the conversion (dither, noise, approximation, etc.) you may be only able to see real bandwidth in the 15kHz range. One must assume that using the line level inputs, a maximum voltage of just over one volt RMS is the upper limit, which would require an input stage with attenuators capable of analyzing the 12 volt plus signal at the output of the speed controller.
On the plus side, the PWM output is around 2kHz and the limited bandwidth would certainly hide brush noise and other switching by products present when looking at motors. You may not be able to see the raw output of a sensor, particularly one that has an output frequency above the A/D. Additionally, the inputs are single ended, where one side of the signal path is computer common. As such anything that is common mode will raise the chassis and ground of the computer to a point that might interfere with normal computing operations. A transformer would be a big help in preventing these currents but needs to be shielded for obvious reasons. There are other scope accessories that can attach to laptop and desktop computers that may be easier to carry and less expensive than a full up scope.