Very nice work bjarnetv.
Just to add on to what Craig has correctly pointed out in regards to pin 1, and your query if it’s something that may be audible. In my opinion a correctly implemented (as per rane notes) balanced line is all about “potential”, in other words one scenario may differ greatly from another, influencing environmental factors are all very different. The only way to test whether a certain environment has audible RFI, would be to dissconect pin 1 from the chassis and listen for hum, in a quiet rural country area you may not hear any as opposed to a city area where RFI is obviously greater.
And then there’s house hold wiring, appliances and fluorescent lights that all add to that “potential”.