KenV:
"Why the open mike all the time"
I might be able to help. As a TRE/TRI for many years I cannot remember how many "loss of cabin pressure" drills I have run in the simulator. Some crews can make an absolute
of the exercise because they are unable to hear one another properly when wearing oxygen masks especially on emergency flow.
I have spent hundreds of hours teaching pilots to use a high level of discipline in the use of intercom in such circumstances. The background noise created when one of the pilots involved keeps an "open mike" is considerable. So, it is vital to use the intercom/transmit switch in the off position unless you have something vital to say and then, as soon as you have said it, switch off again.
This is particularly essential when reading and responding to checklists.
It should come as no surprise that the ex-military pilots came out tops on this front. They were much more used to wearing oxygen masks and had learned the discipline well. The average civilian pilot only ever used an oxygen mask once in a blue moon and possibly never in anger.
As you might have gathered, I am ex-military and in my day it was not unusual to have five crew members communicating with one another with oxygen masks on.