It was a pretty amazing hack, before magnetic memory cores. Because sound moved at a slow rate through a medium like mercury, an acoustic wave (that is, a sound) could be applied to one side of a volume of mercury and be expected to arrive at the other end after a predictable, useful delay. So what would be done is that a column of mercury with transducers on both ends would function as speakers and microphones, which in an acoustic medium are the equivalent of read and write heads!
The system memory would be a collection of these columns, each I guess storing one bit. The memory would of course have to be refreshed: when the signal arrived at the other end, it would be fed back into the column, assuming I suppose that there wasn't a new signal waiting to be written to that bit instead. The article mentions that this was not randomly accessible memory, but rather serially accessible. From that and other bits of information, I gather that the device would visit each bit in sequence, according to some clock, and produce a signal on the read line corresponding to the value in that bit. You had to wait for the memory device to read out the particular bit you were waiting for.
Does anyone know if this a correct understanding of how this kind of storage worked? What a cool way to store bits!