For me, it ended up being the ground loop isolator. Removing that fixed the levels to where I could plainly hear the phone character voices, but (re-)introduced a bunch of noise. I have a replacement isolator on the way from JJP.
P.S. I also played w/ level adjustments in the settings before I did this, and no matter what, the phone character voices were virtually inaudible relative to the rest of the audio.
P.P.S I also tried rolling back to the last full update and then doing an incremental back up to 1.73. Neither step affected relative audio levels.