It is pretty likely that the pipe may be effectively grounded... but so what? The capacitance should be small wrt source impedance even for a constant voltage system.
Without disputing that, what did the poster hear? A roll-off up where only bats can hear? Perhaps a flaky amplifier oscillating and creating artifacts audible to us mere humans.
It's rarely productive to argue that someone else didn't hear anything, we may learn that they heard something other that what they assume they were hearing. I am interested to learn more about this particular situation. The laws of physics are pretty strict but not always obvious.
JR