(maximum of 60 seconds)

is there any $file extension that will do this?

You can use which will return the length in milliseconds. You can use to turn that into seconds. Finally, you can use to return a more readable output.

Note: The $sound documentation says that the file should be mp3, ogg, or wma, but I have tested this with an mp4 and it has return the correct value. So, I would recommend looking up some dll or other alternatives, if this does not work for you.