heck the specs on the outputs of your keyboards. They probably output a signal that's about 20db or so below typical line level (which is what the aurora is expecting) and they possibly have a high impedance (keyboards vary widely in this regard). Most people reccomend that the receiving device (Aurora) have an input impedance 10 times higher than the source's output impedance (your keyboards and modules) for optimal loading..
However, the Aurora "accepts" unbalanced signals just fine. (Input impedance 12khz for unbalanced). So you' should be getting signal into the Aurora, but it might not be at the levels you'd like to see. (Did you try the "-10" sensitivity setting on the input trims of the Aurora?)
I didn't find the right solution from the internet.
-Professional teaser trailer