Sound does very much play a role in Gearboxes or any other bearing function.
How many of us (who drive the same car or light truck for multiple hundreds of thousands of miles) have not at some point heard the "rum...rum...rum...rum" sound of a bad wheel bearing or universal joint?
While flying a corporate MD-500E....I began to complain the MGB just did not sound right....it had changed noticeably to me over the time I had been flying it. For the longest time the CP and Mechanic/Engineer treated me like I was merely going mad. I finally forced the issue and refused to fly the machine unless we pulled the gearbox and did a split case inspection.
Good thing we did....it was in the process of losing some bolts that retained one of the gears. One bolt had worked completely loose and found its way into the sump....without fouling anything, no spalling, no chip detector...just a change in the sound of the gearbox and I am sure a change in its vibration pattern but without HUMs who would have known.
The question I would have on adding a "sound channel" to HUMs....how would one arrive at a "normal standard" for comparison over time? What would be a "significant variation" that would trigger a maintenance action?