On a somewhat related note, the same applies to a few microphones I've had. Not neccesarily the newer ones, which I've been afraid to fry as they're so damn costly when you work minimum wage, but the older little economy ones from the 1970s that bear a resemblance to some retro-sci-fi vibrator. I accidentally plugged one of these into my computer once, and for a week could not figure out why my sound system sounded so weird. Turns out the thing was actually generating sound. Tinny, and horrible quality, but this device, designed for recording sound, was in fact, playing sound. Very weird, indeed.