Bad audio on macbookpro9,2

[John Franklin]

LMDE 2 Betsy (also observed in Mint Cinnamon 17.3)
Kernel: 3.16.0-4-amd64
Machine: macbookpro9,2

Audio sounds find in OS X, but "not right" in Linux Mint when watching a video or listening to music.

When testing audio in the sound preferences the following is observed:

- Output: Speakers (built-in audio) + Analog Stereo Output
  - Right speaker sounds fine
  - Left speaker at 1/2 volume

- Output: Speakers (built-in audio) + Analog Surround 4.0 Output
  - Left Front speaker at 1/2 volume
  - Right Front speaker at 1/2 volume
  - Left Rear speaker no audio
  - Right Rear speaker full volume

The following Pulseaudio packages are installed:

ii pulseaudio 5.0-13 amd64 PulseAudio sound server
ii pulseaudio-module-bluetooth 5.0-13 amd64 Bluetooth module for PulseAudio sound server
ii pulseaudio-module-x11 5.0-13 amd64 X11 module for PulseAudio sound server
ii pulseaudio-utils 5.0-13 amd64 Command line tools for the PulseAudio sound server
ii speech-dispatcher-audio-plugins:amd64 0.8-7 amd64 Speech Dispatcher: Audio output plugins

What other information would be useful?