Comment 1 for bug 368121

Did some more research. /etc/mpd.conf now reads

audio_output {
  type "alsa"
  format 44100:16:2
  name "audio out"
  period_time 50000
  auto_resample "no"
  use_mmap "yes"
  device "hw:0,0"
}

i.e. lock sample rate at native value bypass dmix according to http://mpd.wikia.com/wiki/Tuning

This has gotten the CPU usage down to around 20%, and made it less spiky. I've been able to drop audio_buffer_size to 4192 and max_output_buffer_size back to the default.

Might try OSS as a regrettable workaround.