Well if they did not take the time to label it, how important can it be?
However as for recovering the data, you may be able to use some very expensive equipment that can detect the very minute appearance of the old recording. This requires expensive gear and an even more expensive person that knows what they are doing. And at best they will be able to capture some of what might have been said most likely not even half. Also if this tape had been used prior to the sweet 16 and has been record over something else, you will have even more noise to try and filter through.
Chances are the data is lost, unless you have some serious funds. Even then it will not be anywhere near the same as the original.
If something is important atleast take 5 seconds to label it.
P.S. this is very different from recovering digital data that had been deleted or even over written. Analog is so much less forgivening.
