I think if you talk to responsible motorbike riders, you’ll easily find people who have had near or actual collisions because because a car didn’t see them and the driver wasn’t trying hard enough to check for the places a motorbike might be. It’s easy for a motorbike to be hidden behind the A-pillar of a car if that car is entering a roundabout and the bike is going round, for example, so if a driver isn’t paying enough attention and moving their head to see round blind spots, they can hit a bike. Similarly for any manoeuvre that involves crossing lanes: drivers often don’t notice smaller motorcycles or scooters (or cyclists).
This happens regardless of the noise the motorcycle is emitting though. In slow traffic, the advantages of noise are low. At speed, you can't hear the engine of the bike before its too late.
Cars are way too well isolated nowadays for the noise to be a major factor in collisions with bikes in my experience.
I’m not trying to claim that the loudness theory is correct. If I try to imagine what it’s like to be a biker who has had one or several near-misses/accidents due to careless drivers, whose biker friends espouse the loudness theory, it’s pretty easy for me to see that person feeling a louder bike would be better.
