I have a full copy of the Passware Password Recovery Kit which can find passwords in many files - but it can take many years (!!) of number crunching to achieve it, especially for the more recent versions of MS Office.
If you send me the file I will do an initial assessment on it free of charge which will give a trivial / easy / hard / almost impossible view of the task.
Do you know anything about the style of password used e.g. capital vs lowercase, letters vs digits vs special characters, approximate length ? Or do you have any other files (preferably older) that could be protected with the same or similar password ?