I can't see any other explanation than that the gain of the Akai/the other interface simply doesn't go to zero or there's some kind of a leak. I can't imagine anything could be done in SONAR to fix it.
Another thing is, what problems does this cause to you?
In what kind of situations does it disturb you? Does the same happen with vocal/microphones?
I've never tried if my E-MU lets anything through with input gain at zero.