Cable impedance is of almost zero concern in audio work. Coaxial cable = shielded cable. Sometimes you have to disconnect ground on one end of a cable to prevent ground loops or pickup. I find grounding is most important for low distortion measurements, and the best method is not always obvious. Many times twisted pairs will work just as well as coaxial.
In crazy-nutvolter land, currents are usually low unless it's a current measurement actually being made. OTOH, voltages are usually high enough to be out of the noise. In crazy-audiophile land I find the more common problem is having enough scope gain to see the things I care about, signals on ground lines. An old scope with a differential plug-in serves me better than the latest digital thing. It's nice to be accurate, but anything beyond a couple decimal places is usually overkill. Exceptions might be power measurements, where the result is a squared function, or RIAA accuracy, which is much harder to do well then most people think. An adjustable precision ratio transformer comparison is probably best, but not many people have those anymore.
Audio is my first love and there's a lot of capacitor and other info on my web site that's geared to audio.