When doing fetish ball demonstrations back when I used to do the "club scene" circuit, the dominant would ussually get right up into my ear and speak to me just fine no matter the music level (but then we never ussually did anything there that we didnt allready do in private somwehere else first).

During "casual play scenes in the clubs" Ive seen in lieu of talking, some people use hand singnals, others have the submissive hold a ball or some other such object (if the ball is dropped "scene" over etc), and a few rare others have developed through experience the skills to read non-verbal reactions so well that they simpley know when enough is enough. (especially when with the same partner for a long time, though Ive seen some who could read in that fashion in general as well)

The so called "safe word"...alltough a useful tool (especially for those new to bdsm)...should never be the end all be all "crutch" that a lot of people seem to think it is. It can be unreliable at times, especially when a submissive is in a diminished capacity to respond, or experiencing phenomena such as "sub-space" etc, hence why one should never "just go until they hear it uttered".

Besides no safeword system should remove the onis of responsibility from the dominant partner.