I recently moved into an apartment with both my dogs, Romeo and Juliet. A
very quiet neighbourhood with alot of babies. My upstairs and downstairs direct neighbours all have newborns.
My dogs are barky in general and would alert me whenever my neighbours came home or if someone was at the door or if there was a weird noise. Repeatedly. Until I shush them. Juliet is more sensitive to noise than Romeo, but when one barks, the other will chime in.
Juliet has some separation anxiety too, and has been heard howling before when I left her with Romeo alone in the house.
Being conscious of all this, I made a point to do what I can to reduce their anxiety and manage the sounds they'd hear from outside.
I tire them out physically the night before (I am not a morning person and can't do morning walks!) with walks or training or whatever. On weekdays, they'd be so beat they can't wait for me to return to work so they can sleep for 2 days straight.
Before I leave the house, I turn on the stereo and play all 3 volumes of "Through a Dog's Ear" (classical music designed to calm canines apparently), close up the house (but leaving a fan or the aircon on for ventilation), and just before I step out, I give them a stuffed frozen kong. By the time they are done with their treat, they've gotten over me leaving, and just fall asleep.
I watch them on webcams I have in the house, and they actually don't bark as much (when the neighbour returns) as when I'm around. That surprised me.
I have DAP spray and the wall plug in diffuser on the way in case as well, but from what I'm observing in the cameras, they seem to be doing just fine.
When I'm home, I condition them to accept sounds they might hear (construction going on nearby, the lift going 'ding', etc). And I train them to be quiet on cue. And I use that cue often (rewarding often also) when they decide to tell me (repeatedly) that they heard something weird.
It's harder in an apartment with shelties but not impossible - you just need to be conscious about it and actively train the dogs.