Hey all, this is a quite old topic, so I'm replying to it a month late, however I want to clarify a few things.
I have tweaked the volume of the announcements and the passenger ambience for it to be a realistic level. I won't be adding a slider because imo for sound it should be as realistic as possible, and the volume will be generally realistic for these sounds.
The cockpit door is able to be "opened" even though it doesn't show it visually. If you use the door lock switch on the pedestal, set it to unlock and you will hear everything like passengers speaking, and flight attendant announcements.
Now for people who have volume sliders all over the place in msfs settings - set EVERYTHING to 100, other than music/voices as those do not matter. This will give you the most realistic soundscape and cause the least issues. For EFB sliders those should be set to the 0 (middle, default) value to get the most realistic soundscape, except for exterior master as that is all custom.