I can confirm that the package in proposed fixes the problem for the
Plantronics DA40 adapter.  I've never been able to reproduce the problem
with my DA45.  Jan, you may want to also verify that this fixes you
DA45.

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https://bugs.launchpad.net/bugs/416190

Title:
  Pulseaudio produces overdriven audio on mono downmix (e g to
  Plantronics DA40 adapter)

Status in “pulseaudio” package in Ubuntu:
  Fix Released
Status in “pulseaudio” source package in Precise:
  Fix Committed

Bug description:
  [SRU Justification]
  Impact: See below. But in short, distorted sound when on an analog-mono 
profile. Most soundcards support analog-stereo only, but for some hardware, 
this is a problem.

  Testcase: Using hardware that supports analog-mono profile, play back audio, 
source
  material at sufficient volume. If this bug is fixed, there is no distortion.

  Regression potential: Can't think of any, and if so, it should only
  affect downmixing to mono, which is unusual anyway.

  
  In Intrepid and Jaunty, audio sent through the Plantronics DA40 is distorted 
if the per-application volume is above around 80%.  Unfortunately, all 
applications default to 100%.  Audio sent directly to the ALSA device is not 
distorted, so this is something happening in pulseaudio.  I attempted to test 
this under Karmic, but the volume sliders behave very differently, with the 
pavucontrol sliders adjusting the ALSA "Speaker" mixer, and I was unable to 
remove the distortion at all.

  The DA40 is a mono device, so perhaps it's a problem with pulse
  downmixing stereo audio to mono without reducing the per-channel
  level, resulting in clipping.  but I tested with a DA45, which is a
  similar mono adapter, and the problem didn't exist.

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