HT4619: iMac (Mid 2011): External features, ports, and connectorsLearn about iMac (Mid 2011): External features, ports, and connectors
Currently Being ModeratedMar 24, 2013 3:12 PM (in response to Santivin)
Welcome to the Apple Support Communities
First, try connecting and disconnecting headphones again, so your speakers should be working as usual after doing it.
If it doesn't work, try resetting the PRAM > http://support.apple.com/kb/HT3964 This will reset the speakers information, so the speakers should work again
Currently Being ModeratedMar 24, 2013 3:13 PM (in response to mende1)
Sorry. The link to reset your PRAM is this one > http://support.apple.com/kb/HT1379