What happens to audio?
It depends entirely on how a channel has been configured and the nature of the network used to convey its audio.
In the case of multicast, users connected to different clusters will still be able to hear one another provided the multicast network used for voice traffic has not also become partitioned somehow.
In the case of unicast (Rallypoint) traffic, the behavior will depend on the design of the Rallypoint mesh. In the simplest case, a Rallypoint is present in both clusters and together they are meshed. If a channel is configured to use this cluster-to-cluster Rallypoint mesh then channel audio will likely be partitioned in the same way as provisioning messages: Users of Site A will be able to hear one another and users of Site B will be able to hear one another both a Site A user will not hear a Site B user and vice versa.
Note: There is no relationship between the ICE Server and Rallypoint utilized by an end user’s device. A client device could be connected to Site A for ICE Server provisioning messages and also to Site B for Rallypoint traffic.