A deleted file cannot be retrieved...

What you may do though is to back up the file prior to deleting it:

Backup files are saved in a folder created for each file download date in the "Backup" folder. The backed-up dictation file will be automatically deleted 90 days after file download.

If they recorded on the smartphone app, it might still be in their Sent folder or Recycle Bin.

If they recorded on a recorder, the dictation file downloaded from recorder is temporarily saved on the PC and saved for a certain period just in case. For the save location of the file, refer to [General] -> “Data Folder” in <6.3.5 Settings Menu>.  This information can be found in section 6.5.2 Backup of Dictation Files, in the manual.