Instead of having "in" and "out" sounds, how about attaching a "sound collection" to a keyframe with a play chance. So you would have a keyframe with a chance to play one of the sounds in the "in" collection and a keyframe with a chance to play one of the sounds in the "out" collection. Ditto for slaps.
Allow users to define their own collections from an online library of available sounds, so if they don't want a particular "slap" they can just exclude it from their collection.
Sounds that always play can be 100% chance to play from a collection containing only one sound.
This would let us add moans, bed squeaks and other sound effects. I imagine all sounds would need to exist on a server somewhere (like YL interactions) and would be downloaded along with the interaction as needed. Sounds that have been removed from the server for "reasons" (maybe someone uploaded a sound they ripped out of Skyrim) would just not play.
The community could contribute to the sounds library (like FreeSound.org) and a moderator could give all contributed sounds a quick listen to make sure it isn't just some guy yelling obscenities. There would also need to be a way to report sounds that violate the rules -- maybe the moderator doesn't recognise a particular sound as being ripped off from an anime or something, but someone else does and could report it.