Because of the nature of the problem, it's really difficult to know what is going on here. Either
- The issue is batch or model related, which explains why some users hear the noise and some don't. So far we've ruled out Samsung vs TSMC chips as being the differentiator. We don't know if it differs by cellular radio model (CDMA vs GMS, etc.), but I've gotten reports from people in Asia Pacific, UK, and US (AT&T).
- The issue is design related, and the reason why some users don't hear the buzzing is because they are lucky enough not to have any earphones that can pick it up or they aren't listening in environments quiet enough to hear the noise.
I think one way we can determine if it's batch/model related is to find someone who is using the 6S with a pair of earphones that we already
KNOW should be picking up the noise quite obviously in a quiet room, and who
can't reproduce the noise. So far some earphones identified as being particularly sensitive to the noise are UE11 (chowk), ATH-IM02, NuForce Primo 8 (me), Heaven V (vwinter), UE600 (yuriv). It's also been told by one Youtube commenter that the noise is apparent on the Shure SE530 but not the UE900, so it's not as simple as my original generalisation that it could only be heard on multi-BA earphones.
If we can find someone who can't reproduce the noise even when we are pretty sure they should, then it would suggest its specific to certain units and we can start to work out what units are affected and which aren't. If we can't find someone who can't reproduce the noise with the same earphones, then it starts to suggest its a widespread thing.
Personally since I tested multiple Apple store units and picked up the buzzing on all of them, I'm leaning towards it being more widespread, or at least related to cellular model.