Answer by pLumo for Convert .xls/.xlsx spreadsheets to multiple .csv's based on a list

csvkit version > 1.0.2 has a builtin function to write all sheets:

--write-sheets: WRITE_SHEETS
                      The names of the Excel sheets to write to files, or
                      "-" to write all sheets.

So you could try the following:

find . -name '*.xls' -o -name '*.xlsx' -exec in2csv --write-sheets "-" {} \;

Note:

This seems not to work 100% as expected. But worth a try and as this is the first version with that option maybe in future versions the implementation is better/easier.