Question

How will you mark something that you can’t hear in the audio file?

Answer 1

While transcribing if you cannot hear something in the audio file you should mark it as [inaudible]
There are different style requirements for transcription. Transcription can be Basic or Verbatim. There are a number of tags which are used while transcribing. For example tags like [laughter], [phone ringing], [applause], [music] are used to indicate what is going on in the speaker's environment. 

Hope it helps....

Answer 2

while transcription if you cannot hear something in audio file you should mark as {[inaudible]