Find original Hz of converted audio-file?
The short answer is no.

The longer answer is that if there is some known signal that you know the frequency you could but this is knowledge you bring to the problem.

You can look around 60Hz or 50Hz depending what the utility power frequency uses and see it there is some suggestion of a steady line in the data in that frequency range.

Musicians, after thousands of hours of playing an instrument will know what it should sound like.  While somewhat subjective, musical aesthetics are more than adequate for aesthetic purposes.  

Summary,
If all you have is a file, not a chance.

If you have some clues and hunches and familiarity with the data there is a reasonable possibility.