this happened the last time we were with our friend B. as usual, we met at a hotel on a weekday afternoon. we were all well aquainted with one another by this point, so there was very little social time and more sex. they were kissing and he was undressing her, as I picked her clothes up and folded them, I turned around an he was already inside her. guess he couldn't wait because she still had her shirt on!
anyway, an hour or so into the festivities, my GD work phone kept ringing. when it was obvious that it wasn't going away, I took it into the bathroom to see WTF was so important. I didn't pay much attention, as I was really listening to them fucking. he was grunting and making an ******* like moan, I could hear my wife's moans, and occasional gasps for air, so I knew he was kissing her deep while he was in her. then all of a sudden the pace and tempo really picked up. I could hear his balls slapping her ass really hard and she was getting louder with each smack. I got off the damn phone and came out just in time to watch him cum in her for what seemed to be the longest time.
may not sound that great, but it was my first experience not seeing, but hearing only. it was utterly fascinating and erotic to me.