I noted in a February 20, 2016 posting that the 6D build (August 2015) did not interface with the Sescam DSLR-550D-HOCF A/V Out Cable. In reading entries on or about October 14, 2013, it appears a developer (1%) released or compiled a source code at
https://bitbucket.org/OtherOnePercent/tragic-lantern-6d/downloads/magiclantern-Tragic.2014Jan03.6D113.zip to allow 6D audio headphone monitoring using the Sescom A/V Out Cable.
It appears the latest 6D build (May 2016) did not incorporate the source code to fix the audio monitoring using Sescom A/V cable. I await any suggestions to fix this issue.