Assuming
@ClassG33 is in the US and not Europe, that does not support the idea it's 60Hz hum. However, this is one of the weirdest things I've heard of. It is not an ordinary occurrence, with ordinary answers! So we should keep open minds and not jump to conclusions.
I suggested the radiated energy may be at a much higher frequency than we hear, and the audible tone is a demodulation of the high frequency carrier wave, like an AM radio. So let's explore that hypothesis a bit.
We have transfer of energy over a distance of several feet, enough to make an audible sound in the speakers. It is hard to transfer energy at 60 Hz magnetically. 60Hz transformers need steel or iron cores to give them the large magnetizing inductance they need to transfer power at 60Hz. Air core transformers are very inefficient at 60 Hz. And the buzzing speakers are separated from the amp by feet of air.
Wireless charging can be performed over an air gap. To address the inductance problem, the frequencies are much higher - on the order of 100's of kHz. At these frequencies the required inductance is much lower and air core transformers can be used. Typically these are formed into resonating tank circuits with a capacitor. So it seems possible that the energy is being transferred from the amp to the speakers by a high frequency EM field rather than a low frequency one, as appreciable energy can be transferred through relatively small inductors, such as those in the LC networks of a speaker crossover.
An amp can generate high frequencies if it is oscillating. The joke in Analog Design 101 classes is that if you want to design an amplifier you get an oscillator; if you want to design an oscillator you get an amplifier. High gain amplifiers will oscillate without careful control of the phase and gain in the feedback loop. Certain defects can cause a loss of feedback control, resulting in oscillation. For example, many amps rely on a single capacitor to establish the "dominant pole", which attenuates the gain with increasing frequency, so the amp runs out of gain before it reaches a frequency that it becomes unstable. A bad solder joint on that capacitor and the amp can oscillate. Oscillation can be expected to occur around the unity gain frequency (the frequency where the amp gain = 1), which is often in the 100kHz range or higher for high performing amplifiers. So there is the theoretical possibility that the amp is oscillating and transmitting RF power in the longwave or maybe even AM band.
How can that produce sound in the speakers? The coils in the speaker, both the inductors in the crossover and the voicecoils, can convert the alternating magnetic field into AC current. There are several current loops internal to a multi-driver speaker with crossovers that the current can flow through, through voicecoils, inductors, and capacitors. So no Kelvin's laws need be violated in the production of the buzz.
To produce an audio frequency from a carrier, a demodulating mechanism is required. But there are several 2nd order distortion mechanisms in speakers - and those involving eddy currents are likely to be very strong at high frequencies. Second order nonlinearities can demodulate the carrier.
Last for the source of the 50Hz, or 60Hz, or whatever frequencies show up in the spectrum. If the oscillator were producing a steady sine wave, there would be no sound in the speaker, even if the HF was received, converted to current, and demodulated. But unintended oscillations can be messy. They can pulse on and off, as the large amplitude signals shift the bias points, which change gain and capacitance in the devices. They can oscillate chaotically, which will produce a broad spectrum of frequencies. They can oscillate at some mix of frequencies, with intermodulation components.
So maybe there is a mechanism to produce sound in a disconnected speaker at a distance from a high power oscillation. A lot of things would have to happen for this to occur, and it is not a likely scenario. But
@ClassG33 has speakers buzzing with no electrical connections! Something very unlikely has already happened here, so anything goes.
How to test this hypothesis? Easy if
@ClassG33 had an oscilloscope, which I presume he does not. Do you have a portable AM radio,
@ClassG33 ? If so, does it exhibit any strange behavior around the haunted amp? Try sweeping the dial and see if it picks up any static or buzzing.