The easiest way to find out would be to print the pages for the PSR-S970 voices and the PSR-S770 voices from the Data List, place the pages side-by-side (S970 to one side and S770 to the other), and see which voices are listed for the S970 but not for the S770. This might sound like a lot of effort, but the voices are arranged in the same categories and order for both keyboards, so it should not be difficult at all to place the corresponding pages side-by-side for comparison.