You would have to give the context. It could refer to sound. "He heard the flop of letters hitting the desk." 'Flop' here refers to the act of letting something drop with a great deal of noise. If you flop down in the chair, you let yourself drop into the chair, with the muscles limp. You would make more noise that way.

Where exactly did you hear/see this? I'm not sure I have heard the word flop used in this context. It sounds like you are using it as a unit. The only time I've seen it used in terms of units is in computers and even then it is not really a word but an acronym.