Since upgrading to the new 3.0.x kernel series (via an upgrade to Oneiric), the sound in my Thinkpad R61i laptop has started to have problems.
  1. Headphones. The headphones socket doesn't have any output. The computer speakers are muted, but I can't hear anything through the headphones. I have checked that the volume levels in ALSA Mixer haven't gone to zero (as I have read this is/was a problem for some). The computer appears to be playing the sound just fine, and if no headphones are plugged in then I can hear the sound, but the headphones don't work.
  2. Volume level. The volume scale seems to be wrong - below about 85% I can hear nothing, the sound is completely muted. It only works between 85-100%.

Neither of these occurred in Natty with the 2.6.x kernels. I also tried using a 3.0.x realtime kernel before upgrading, and if I booted the 3.0.x kernel then these issues would appear in Natty as well - so I believe this is a kernel issue and not something to do specifically with Oneiric.

What should I try and do to establish the cause of the problem? What changed in the new kernels with respect to sound?