On occasion, I used to hear a certain 'hiss' on the strop, when the edge was getting very sharp, and the angle was just so. Unfortunately, my ears aren't quite up to that anymore. Combination of age-related hearing loss (or so I assume) and some ringing in the ears that just seems to get louder, the older I get. I tend to believe, the audible feedback depends a whole lot on too many variables. Thin blades at high hardness seem to 'sing' a bit more than a blade that's either very thick, or not quite so hard. Coarseness of the finished edge would make a difference too, I suspect. And then there's variation in the surface characteristics of the strop itself (rough leather vs. soft).
I have found, with some experience, it's fairly easy to 'feel' the correct angle on the edge of a V-bevel. If it's perfectly flush on the strop, it tends to be very 'slick' feeling. If the angle's too steep, the edge will drag and will consequently feel rougher. Same deal if the angle's a little too shallow, the shoulder of the bevel will drag and feel somewhat coarse/rough. Convex bevels aren't quite so obvious at revealing where the 'shoulder' is (obviously), so the focus is kept on feeling for the edge itself, if the angle gets too high.
If the sound is working for you, that's a bonus. So much of the 'art' of sharpening comes down to the (nearly) intangibles, some VERY subtle cues that not everyone will immediately see (or hear, or feel).
