OK, and if my second hand goes in at a node as well, I guess you want six sounds playing?
It sounds like you need arrays to make the program practical: to hold the status of whether each note is currently being played or not; to hold the code you need to send to MIDI for each note; and maybe in other places too.
You could set up 2-dimensional arrays (8 x 8 elements) to hold the data that relates to the 64 nodes. For example, the booleans that say whether each node note is being played or not.
But you also need data that just relates to the rows or the columns. For example, the booleans that say whether each single note is being played, which is independent of the node notes. You could set this up as separate 1-dimensional arrays (a 1 x 16 or two 1 x 8s), or you could use the first row and column of the 2-D array if you make the array 9 x 9. So row 0 and column 0 have this special purpose, then rows 1 - 8 and columns 1 - 8 relate to the 16 sensors.
On the hardware side, there are only 6 analogRead capable pins on a Uno, I believe. It should be possible to configure your sensors so that the voltage going to the Arduino can be read on a digital pin as high or low, which is all you want in this application anyway.
Regards
Ray