I keep my T500 current with new drivers, however in a recent update (I do not know which) I lost almost all the volume on my speakers and headphone.
T500 series running Windows 7 64bits
The problem can solved with a full power reset of the laptop.
Shut it down and take out the battery,probably out overnight.
Plug in the battery back, switch the laptop on and it and you must not be getting the sound level as usual.