FCOM verbatim from an affected aircraft:
In the case of an ELEC DC ESS BUS FAULT, the audio cards of the AMU are lost. Since all cockpit
mikes and headsets are connected to the AMU, all audio means of communication (VHF, HF and SATCOM )are lost.
The use of VHF 2 or 3 do not allow to recover communications.
Not sure how the RAT would help things if you have a bus fault?