My roommate's bedroom shares a wall with mine, and when she clunks stuff around I can hear it. She and her boyfriend will often spend large amounts of time in there with the door closed, frequently ending in him sleeping over. She's got a jumbo bottle of Astroglide sitting out in the open.
And yet... I never hear sex noises. Ever. Now, despite how this post probably sounds, I don't exactly stand there with my ear to the door. But gosh, sometimes I hear the neighbors in other buildings having sex; the soundproofing's not that awesome here. While I'd probably be disturbed if I had to constantly hear my roommate screaming like a champ, I think I'm more disturbed that I've never heard so much as a sigh.
I guess they just always have silent sex? Is that common? I'm physically incapable of operating at a level lower than "sexy Saturn V launch" and it disheartens me tremendously when my partner doesn't vocalize a little. But maybe if you're both silent that makes it okay?
It's things like this that make me understand why I'll never be a "sexpert."