For me, it's Alexa/Amazon Echo smart speakers.
For some reason the intercom feature doesn't seem to work properly on my setup, so instead we use announcements. With announcements, whatever you announced is played back both on the speaker you were talking to, and through the other speakers on your network. I get to hear, straight away, how the things I said would've sounded to another person, far more than I ever have before.
I know almost everyone dislikes recordings of their own voice ("ugh, do I really sound that nasal/high-pitched/childlike/whatever?"), and I'd guess I'm not that weird to feel an initial cringe at hearing Alexa apparently replace my voice with that of a congested preteen with an accent far stronger than my own 
But I've also realised, from hearing myself on announcements hundreds of times, that, not infrequently, my tone comes across entirely differently from how I intended it.
I think that I'm just speaking quickly so Alexa doesn't time out before I'm finished; I'm actually projecting irritation.
I think I'm emphasising a couple of key words to make sure they come through in a dodgy recording full of background noise; I actually sound condescending, exasperated or even pissed off.
I think I'm telegraphing the lightheartedness of a comment by injecting plenty of pitch variation and ending with an upbeat tone; in reality, there's barely any perceptible difference from me telling someone their dog's died.
(Before anyone asks, yes, I've been diagnosed with ASD, but — just like I have with a lot of other aspects of social norms, behaviours and etiquette — I've put a lot of effort into trying to learn to speak in a way that communicates what I want to communicate. Unfortunately, unlike with facial expressions, I grew up without a mirror for my voice This is clearly something to work on further…)
Has anyone else learnt something about themselves, maybe something they weren't expecting, from using some kind of technology?