I think that, in my personal case, the realism of sound has a lot to do with scale. And the awareness of selective compression and forced perspective plays a big part of that for me.
An easy(?) explanation: Place an O-gauge loco, an HO-scale loco, and an N-scale loco next to each other on a table. Stand back 10 feet or so, then sit or stoop down at eye level, and look at them. Now, with selective compression and forced perspective in mind, imagine each one being a
real locomotive, and ask yourself, which one would I most likely hear running if I were that far away from it? Which one would I most likely NOT hear running, if I were that far away from it? And which one may or may I not be able to hear all that well?
Personally, I have no use for sound in N-scale. Maybe cute for a minute or two, but for me, it gets quite annoying in short order. I can live without N-scale sound altogether. HO-scale is kinda' the same way. I can handle it a little better than N-scale sound, but after awhile I can turn it off too and still be quite happy. For instance, being on one side of a basement layout, and hearing the HO-scale sound equipped loco way over on the other side of the layout almost as clearly as if I were still standing next to it just doesn't seem right.
O-gauge/O-scale trains are another story. I have no problem at all with sound in O-gauge, because considering the scale and one's perspective, you're never really
that far away from the loco to begin with. You would almost expect to hear the sound coming out of it, no matter where it's at on the layout. So I find it much more believable, and therefore enjoyable, to have sound-equipped O-gauge locos. And still, I'm also fine with shutting off the sound or running my non sound-equipped locos to boot. I don't always have to have the sound on in order to enjoy watching them run. Mainly because running a string of O-gauge cars behind your locomotives makes a fair amount of racket the way it is, with the metal wheels pounding on the metal track and clacking at all the rail joints and banging thru the turnouts and crossings and drowning out the sounds of the locos, just like the real McCoy!
Well, that's my take on my situation. Others will have other takes based on their own personal preferences. As long as YOU enjoy whatever you find enjoyable with (or without) sound, that's the main thing!