Just ran across this post in a search for the schematic for this board. I thought that perhaps it was closing each button to ground, but it's more complex than that. I used a multimeter to try to figure out what's going on. Here's what I found so far.
There are 4 distinct circuits that connect the 5 button centers and 5 rings. If you label the buttons:
NX = next file
PV = previous file
VU = volume up
VD = volume down
PP = play/pause
IR = inner ring of a button (really a dot)
OR = outer ring
Then they seem to be connected as follows:
Circuit A connects PV-IR, PP-IR, and VD-IR
Circuit B connects PV-OR and NX-OR
Circuit C connects VD-OR and VU-IR
Circuit D connects VU-OR, NX-IR, and PP-OR
Circuit C seems to be a pull-up to Vcc
Circuits A, B, and D have pulses or pulse trains with a frequency of about 100Hz
If you look at them as a matrix, then pressing each button would connect the following circuits:
PP: A->D
VU: C->D
VD: A->C
NX: B->D
PV: A->B
I'm interested in controlling this with a Nano or other micro too.