Why is speech malformed when listening to a USB signal with an LSB receiver?
The spectrum is reversed. Nice images at Wikipedia 


LSB is short for Lower Side Band and 
USB is short for Upper Side Band. 
Normal AM transmissions are Dual Side Band, 
but with filtering you can create Single Side Band (either LSB or USB).


Basically with AM modulation the audio spectrum is 'shifted' right with an equal amount as the carrier wave to the USB and then mirrored from USB to LSB, with the carrier as mirror.

Or you can reason that negative frequencies the give an identical signal as positive frequencies and hence when you start to shift it up the spectrum by amplitude modulation you get two lobes, USB and LSB.