I recall, One bit sound was detailed extensively in Byte Magazine starting in mid 70s. A square wave that has its frequency varied. A google of the words " Generating speech with a square wave wave , Byte magazine " picks up lots links about this technique. Fourier transform show a square wave consists of odd sine wave harmonics. [A more advanced technique (more than one bit) would use microprocessor interrups to generate several sqare waves, output them with more than one IO pin and combine them with a resister network]

This mention of one bit sound made me remember the Lis’ner 1000 project of Steve Ciarca , cira 1984 , WHICH WAS speech recognition system of about 30 words. It used a GI SP1000 chip