Flashing white, or flashing white then orange?  If the latter, the Amp is in a fault condition due to overloading.  The impedance matching controls should prevent this, but if you have high impedance speakers (4 Ohms or under) it may be struggling to power two pairs.  

If it’s flashing white, it is going into reboot mode and may have a hardware issue.  Send a diagnostic to Sonos and post the reference number here (or contact them directly with the number). 

Check silly things, such as accidentally mixing input and output as you wired the Volume controls. One or more controls could be set for the wrong impedance. Make sure that the wires are tidy and there is no possibility that bare speaker wires can touch each other. Make sure that the common (-) terminals on the Volume controls are connected to the common (black) terminals on the CONNECT:AMP and that left and right common terminals are not connected to each other.

A more esoteric issue could be that there is a wiring fault along the way and a bare speaker wire is touching a metal surface.

To add some more information to this, the speakers have 6 ohm impedance so not sure if the amp can handle two pairs. I will check my wiring to make sure it is correct and there are no bare wires touching each other. 

With two pairs of 6-Ohm speakers, this would be the expected behavior when the Volume is too high. In any case, check the wiring.

Impedance matching Volume controls for each pair will resolve the issue.