I have a ~year old 24" iMac (2.8 GHz Core 2 Duo) that I just suddenly started having a problem with. I use headphones pretty regularly and they are almost always plugged into the headphone jack. Recently, I lent my headphones to someone and, when I plugged them back in, the machine doesn't detect them. The audio works just fine coming out of the internal speakers, it's just that it *always* comes out of the internal speakers, no matter if the headphones are plugged in or not. I took a look at the Sound Preferences panel and, sure enough, it doesn't detect the headphones as plugged in (just the internal speakers). Does anyone have any suggestions on troubleshooting? I'm not sure it matters, but I did just recently upgraded to 10.5.6. The headphones worked after the upgrade, but it may be that this is the first time they were *re*plugged in after the upgrade.