I'll assume this really is for a noble cause.
There is no easy way to retrieve a password of this sort, but there are ways. My favorite programs for recovering passwords within files are from a company called Elcomsoft
, they are not free, but they do work.
For paswords within files such as this there is little more you can do then to do straight search, because there is no real pattern within the file that determines where the password will be. The program you are looking for specifically is: Elcomsoft - Advanced Office Password Breaker
depending on the size of the file this could take up to a week to complete... a long time, but if you really require this password it is the best way to go.
This program comes with a few nice features allowing you to stop the process and start again from the block at which the program was stopped. It also does no use that much memory so you can go about your daily computer routine while it works in the background.. unlike a few other programs i've tried.
I do not know of any reliable free programs that do this, so you may be stuck with this route.
-Hope this helps