You can load a pre-recorded audio file and then transcribe the recording. Currently, you are limited to five hours of transcription per month for uploaded recordings.

1.       Verify that you are logged in to Microsoft 365 using the new Microsoft Edge or Chrome.

2.       Go to Home > Dictation > Transcribe.

3.       Select load sound.

4.       Select an audio file from the file selector. Transcribe currently supports the following formats: .wav, .MP4, .M4A, .mp3.

5.       Transcription may take some time depending on the speed of your Internet, i.e. the length of the audio file. Be sure to leave the transcription pane open while transcription is occurring, but feel free to perform other tasks or to switch between tabs or applications in the browser and return to them later.

Source : Support Microsoft