Fairly old one.
Man comes home one night, and finds his wife arse naked in bed with another man, with his ear on her left breast.

Cue the husband shouting, "What the f*** are you doing?"

The guy replies, "Listening to music."

The husband, confused & curious, puts his ear on her right breast and says, "I can't hear anything."

"That's because you're not plugged in."
(Purple Jonax, Mon 6 Mar 2006, 14:36, Reply)