A "group sound bath" is similar to a small musical performance, except everyone is lying down relaxing, and the instrument is a bunch of stone bowls that make deep droning sounds when you run the stick thing that they come with along the rim of the bowl. The sound production is very similar to the way a crystal glass sings when you rub your finger on the rim of the glass, except far deeper in tone because of the large size of the bowls.
Witchy-types believe there is some sort of healing/metaphysical effect to being exposed to those sounds from those bowls, but without speculating on effects that cannot be measured I can only attest that the single sound bath I've attended was incredibly relaxing.