If something can't be distinguished in an audio file it can be marked as [unintelligible] While transcribing if you do not understand something because of the speaker's way of speaking or accent you can mark it as 'unintelligible'. If you cannot hear anything because of the background noise you can mark it as 'inaudible'. You can use other tags like 'laughter', 'music' and 'applause' to describe what is happening in the background of the speaker.
