I have the same problem - and it is recent. I have been using Mint 7 x64 since it came out, and ASAIK the sound problems started around the same time apt-get upgraded to the 2.6.28 kernel. If I choose the older 2.6.27-7 kernel, I have speaker sound again. I hadn't thought to check the headphone jack plug, but it works.
I do not have a fix for the situation at the moment and created this forum account to see what others have to say.
I suspect this is also the same issue as this thread:
Sound coming from box and through headphonesviewtopic.php?f=90&t=35822&p=206756&hilit=no+sound%E2%80%A6#p206756
lscpi Audio Devices:
01:00.1 Audio device: ATI Technologies Inc RV620 Audio device [Radeon HD 34xx Series
00:14.2 Audio device: ATI Technologies Inc SBx00 Azalia (Intel HDA)
Mint 7 Gloria x64
HP dv2 12.1" laptop
ATI Radeon 34xx series