Update: It is probably wise to try to do a zip repair step on corrupt xlsx files as a first step before trying to invoke other methods.
Microsoft recommends several methods for recovering from Excel corruption. This Excel add-in brings into one toolbar these methods while adding four data recovery methods of its own.
For MS methods for recovering from Excel file corruption, see: http://office.microsoft.com/en-us/excel-help/repairing-corrupted-files-in-excel-HA001034656.aspx or here: http...