@Linda I once listened to this audio once and canould get 80% of it.

Because "once" applies to just the first clause in the sentence, and not to all of it, it makes more sense at the end of the clause.
"Could" rather than "can" because it's not a hard and set ability, and you go on to change that.

When I wasent back to this audio to listen to it again and wanted to write it down, I saw your wtranscriptiong.

"Went back to" rather than "was back to", "was" is already there, no movement. "went back to" is commonly used for "revisited".

If you dihadn’'t written it down, maybe I could stillI still might not understand it completely.

"hadn't written" past perfect because you are talking about something that is completed in the past but relevant to now.
"might" is a weaker conditional, it fits here better, to my ear.

You have perfect listening abilityr listening skills are perfect, and I envy you.