Note that you do have a ground wire in a way. The neutral is grounded at the panel through a bonding jumper in the main panelboard or at the utility meter. What a 3rd ground wire does is provide for a fault path should a short circuit occur: say a short from the hot to your amplifier case. Without a 3rd ground wire, the amplifier case will be energized and the circuit breaker protecting the circuit won’t trip (no low impedance fault path back to the panelboard). That’s the real purpose of the ground wire. Running a conductor to a copper cold water line is against code because the conductors aren’t run together in the same bundle of wire or raceway. A GFCI will provide some life safety protection. Your stereo will be fine without a 3rd ground wire, under a bad fault condition, you might not be.
EDIT: A house without a ground wire is the same thing as an amplifier manufacture telling you to use a cheater plug to eliminate a hum in an amplifier. The same basic safety consideration is at play.