The Airbus flight deck has two loudspeakers. They are mounted on the sidestick console chassis facing the Captain and First Officer sidesticks. Their purpose is to alert the pilot of system alarms and to route audio from the Audio Control Panel (ACP).
If you look closely at the wiring of the speaker, there are four inputs – one input receives audio data which passes through the Lighting / Loudspeaker panel allowing for volume control. The other 3 inputs are directly connected and receive audio data initiated by the FWC (Flight Warning Computer). This allows system alarms to bypass the speaker volume setting as they require full pilot attention.
The ACP allows you to adjust the volume of VHF1 and VHF2 (tuned in with RMP) and the navigation tones of ILS, VOR (either autotuned or manually tuned with the FMGS). Simply depress the knobs on the ACP to activate and turn clockwise to increase volume. The audio is routed either to the Captain or FO headset and also to the appropriate loudspeaker allowing you to individually listen to each specific frequency, or if you wish, listen to them simultaneously.
Several instances of the ProSim Audio application must run to simulate multiple audio streams. System alarms and voice alerts (“retard”) should be selected to output to a sound card which connects directly to one of the 3 inputs of the speaker. For audio managed by the ACP, NAV tones for the appropriate side (Captain or FO) should be selected in the ProSim Audio application.
ProSim radio frequency simulation is realistic with attenuation (volume of the tones) proportional to the distance from the tuned beacon. The closer you are to the beacon, the louder the sound.
Unfortunately at the time of this writing, ACP management of VHF1/VHF2 is limited. Online ATC network (Vatsim/IVAO/PilotEdge) audio should be routed by virtual audio cable or a hardware audio mixing board and then internally managed by the ACP. This feature is natively supported with the avionics software ProSim737 and Jeehell FMGS. The workaround with ProSimA320 at this stage is to output the network client (Vpilot, Altitude) sound directly to the speaker/headset, bypassing ACP management.