A few thoughts…
I don’t have a stethoscope. At around the time I expect 2C I tip the bean chute cover sideways so the underside of the cover reflects sound toward my ear. Works fairly well for 2C; works very well for 1C.
Decaf is very quiet at 2C, but that’s how it is. I have to depend on temperatures and do my best confirm that it’s going on. I usually miss the start of 2C with decaf.
And I have hearing aids which may or may not help since they amplify stuff I don’t want to hear (like the beans tumbling in the drum) as well as the good stuff.
One last one: if you see a few small circular chips falling into the cooling tray during the roast (about 1/8" dia.), 2C is already going on. They don’t always show up on the tray surface but if I hit 2C I almost always find them beneath the cooling tray when I vacuum chaff if I look closely.