"Audio Distortion" is an errant manipulation of sound, experienced in relation to dialogue, music, and or effects, unintentional or has no context within the scene.
The audio is over-modulated resulting in noticeable distortion. Audio Distortion impacts both the Content and the Technical Quality of the Asset. The customer experience is negatively impacted by audio playback containing content that distracts from the overall experience and not typically Creative Intent given the context of the scene.
A minor errant effect within the dialogue, or a major errant sound within music and or sound effects
How to Prevent:
Measure signal in as many points as possible in order to avoid introducing distortion.
Review the original recording and determine if the effect was in the recording or produced during the mix, correct and re-deliver the print master. Check the M&E to ensure that it doesn't exist there, redeliver M&E as well if it is.
If the source sounds ok, then it's possible that the audio levels were adjusted at some point, possibly during ingestion of the tape master (tape deck was misconfigured), during an edit (audio mixer of the non-linear editor was incorrectly adjusted), or during transcoding (a gain filter was applied).
Return to the point in your workflow where you are able to verify the audio was ok and start re-processing from that point. If you have to take it into an NLE (non-linear editing system) to perform an edit then ensure that the NLE is correctly configured. When re-transcoding, ensure no gain filters have inadvertently been applied to the profile being used.