[Johnythree]

This is a myth. There is no reason that channel spacing need limit the modulation bandwidth. The only downside is that listeners to adjacent stations will hear a slight "monkey chatter" from the overlapping sidebands. In reality stations are never allocated adjacent frequencies within the same coverage area so this usually doesn't happen.

[kazinator]

Be that as it may, AM radio is obviously low-pass filtered. It might not be a brick wall at 5 kHz, but it sounds obviously muffled to someone who can't hear anywhere near up to 20 kHz. If I were to guess, based on years of experience of playing with EQs, I would say that it has next to no content beyond somewhere around 8 kHz.

[kazinator]

AM receivers have to apply a band pass filter to select the station. The width of that filter will directly affect the audio bandwidth. The filter probably can't be anywhere near +/- 20 kHz based on the idea that nobody nowhere would put two stations close together in the same area. Mass produced AM radios have to work everywhere.