Find out when an audio file is actually recorded?
Baring information content that contains verifiable historical landmarks, you can't date data .  Anything can be altered in a file, including the recorded content, so the data itself is no more reliable than a time stamp in a file header.  The normal assumption about data files is that they can be faithfully reproduced over and over without degradation, so there is no time information in the data itself.  You may be able to infer rough time estimates about the age of a recording based on certain qualities in the recording (assuming that the data has been unaltered).  For example a digital reproduction of an analog recoding that was originally mastered on a particular media has qualities that may be discernible upon analysis.  If you can identify qualities that indicate the type of equipment used to capture the audio and type of media the audio was captured on, you may be able to infer that the recording is from a certain time period.