I looked through the receiver manual and the remote manual. Nothing in STB setup except that audio can be directed to TV or HIFI.
It is theoretically possible to control volume with remote set to SAT, but it seems it just channels volume control either to the TV or to the audio amplifier. I had it working with TV, but only via the coax connection. Using the composite video feed to TV and audio L/R connected to "HIFI", there is no control of volume. Probably because the "HIFI" doesn't have remote capability.
It's not a huge problem with volume set at a fixed level.