In the Italian community we say this:
Basically, the audio must be correct (text corresponds to the audio and vic.) and you could understand it also without read the text, and the audio doesn’t have mistakes (noises, other voices, …).
Here you can find some guideline about validation: Discussion of new guidelines for recording validation
When you are not sure, I suggest you to Skip the clip 
(I’m not an employee of Mozilla, so these information could be not correct)