There are 2 choices that would indicate an advancement in the technology. Either, there is some material floating around when a sonic shower is used or there isn't any at all.
Without any additional material, the air around the user would be what's used to vibrate the gunk off the body. Or, with their force field technology, they might project EM energy around the user and that's what vibrates to clean them off. An alternative would be that the user's body is what's made to vibrate to get the stuff off. We've all had buzzing feelings before, so it could be something like that.
With before, seeing what looks like water droplets, Picard in TNG: Inner Light mentions atmospheric condensors. That technology could be used to take the air and form it into a fine mist that would be used in the shower to vibrate the gunk off.