"Mono Audio" refers to the duplication of an audio track, effectively eliminating any stereo or surround panning.
Mono audio impacts both the Technical and Content Quality of an asset. If mono audio is not creatively intended, the missing panning effect provides the customer with an incomplete/ inaccurate experience. Any subsequent assets derived from this mix will also be incorrectly mono.
Mono audio can clearly be discerned as creative (i.e. accompanies archival footage where mono audio would be expected or all that is available).
2.0 tracks are identical or 5.1 does not contain any surround panning between channels. Mono is also visually represented on a phase scope, where the audio signal exhibits zero phasing or panning between the left/ right channels.
How to Prevent:
Label tracks correctly in metadata.
Be sure to properly mixdown the 5.1 to derive the 2.0 (do not simply duplicate tracks).
Replace mix with a mixdown of the 5.1 to preserving the panning effects and redeliver. Check the M&E for the same issue, and redeliver M&E as well if necessary.