That speaker-and-stethoscope idea is ingenious. But if that doesn't work...

Put some kind of load that is on and drawing electricity on the end of an extension cord and poke it down all the way to the end of your pipe. You need to have something that causes current to flow down the cord. If you have a long enough cord, you could even double it so that you can have the load on the end outside of the pipe.

Call your local utility locator (usually they are free) and have them come with their detectors and mark it. A hot wire in the ground "should" come up on their device.