@mijostyn makes a good point. If you’re really interested in obtaining a low impedance ground for audio; you might consider your water main pipe as the lower of the two. Of course we all know that our water main ground is connected to the grounding bus bar in our circuit breaker boxes (right?). The main earth electrode (your ground rod or rods) is/are also connected to this same grounding bus bar.
I measured the ground resistance of my earth ground connection on my water pipe, and it was 0.35 ohms, while my ground rod measured 15.0 ohms. I only have one ground rod, but I’m planning to upgrade that to a total of 3 or 4 in the future. What I’m getting at is making your water main coming into your house (for those of us that have city water) a ground priority.
You can make sure this ground delivers ultra low resistance to your circuit breaker box bus bar by making sure it’s connected between the two with a #4 AWG wire or larger (#3 AWG or larger). If you already have this in place - excellent. Making also sure the water pipe ground connection is also clean with little to no oxidation. Improving this water main connection to circuit breaker box is a task I highly recommend you have an electrician perform (if you're a homeowner).