See the Mobhack help (where the technical nuts and bolts tend to be discussed) - Units tab page, EW section, where ARENA and VIRSS is described.
Acronnyms Site useful WWW link, found within 1 minute of entering "virss" as a Google search term. Google is a very useful search site, if you need to find a term you aren't familiar with.
As to the VIRSS smoke sounds replacing the missile hit sound - depends, if the missile was deflected, there will be
no hit sound, as the VIRSS worked
Cheers
Andy