If someone was disrupting the performance, they would absolutely stop.
And do you think they'd stop the performance, go to investigate the noise and be most apologetic to the postman for holding him up doing his work - or do you think they'd be just a little bit angry with him?
I still think that some people on here are interpreting WFH as 'catching up with a bit of paperwork or answering some emails. Sometimes, it may be that, but many, many people work 'live' (as it were) as if they were in the office. You may be calling your bank, get put through to somebody in their 'call centre', but that advisor is actually at a desk in their own home - a virtual call centre, really.
I'm one of the more fortunate ones in this respect - I always WFH and only usually receive/make calls to/from my boss, who is also a very good friend. It's no issue at all if one of us says "Oh, can I call you back in a few minutes - there's somebody at the door" (which does happen on occasion) - but a great many people do a great many jobs (and other things) where it simply is not possible to break off for an unscheduled stop, unless your house is on fire or something.
Incidentally, those suggesting dropping everything important and seeing to the postman's demands: you do realise, don't you, that in so doing, you are validating his behaviour and giving him the confirmation that it's what he needs to do every time he wants to demand somebody's attention - bash your door down and shout threatening-sounding things at you in your own home? Acting very similarly to how you'd expect a violent intruder/aggressor to do so when trying to break in to come and assault you? You still really think HE is being the reasonable one???