Both responses list coiled cords as an example, and how they have lower ratings when wound up then when unwound. This is not due to magnetic induction from the coil, but rather due to the concentrated heating caused by *resistive losses* in the wire, which produce a higher temperature rise when the cable is coiled due to reduced cooling.
At 60 Hz line current, a knot is not going to cause any problems from magnetic induction, and there won't be heating issues since there isn't enough wire in a single knot to make a big difference. A knot will also not effect an audio amplifier's "sound". Any DC-output power supply, such as those used in audio amplifiers, will have capacitors to deal with large surges in current demand and will not be affected by a minute amount of inductance added to the power cord.
Much better than a knot in the cord is to wind several coils of the cord on a piece of iron pipe. The iron pipe serves as a magnetic core which results in an inductor of higher value and is far more effective at stopping large transients (such as those caused by nearby lightning strikes) than a simple knot would be."