Normally when you add a file to your composition, you will be prompted to transcribe the file in the left-hand sidebar, but sometimes you can click out of that sidebar before the transcription is chosen.
Not a problem though, if your transcription doesn't appear you can just hover over the script in your composition, click the dropdown arrow that appears on the right and select Transcribe.
You can also click on the Media Library, locate your file and click on the ellipsis button to the right, then select Transcribe file.