When the Expo. Override option is enabled, frequent audio dropouts occur when recording video. They seem in no way related to any Canon or ML audio settings, and disabling Expo. Override resolves the issue.
The short attached clip demonstrates the problem; the dropouts are particularly noticeable in room noise or any continuous tone.
Currently on magiclantern-Nightly.2018May01.600D102, but I believe the issue dates back to at least December 2017.