I would think the people on the other side of the room, behind a closed door, would have disadvantage in hearing people on the other side unless they were paying attention.
Sure. I said that in an earlier post.

I would think the people on the other side of the room, behind a closed door, would have disadvantage in hearing people on the other side unless they were paying attention.
Surprise in the Starter Box Set is conducted via a Dexterity/Stealth v Passive/Perception test, but that doesn't feel right for a kicking down the door type of surprise; that feels more sudden and less a use of being Stealthy. I guess it could still use Stealth but I'm not entirely happy with it.
What's your take on it?
The PCs not knocking the door down Delay.
There isn't a delay action in 5e. Not having that action kind of messes up surprising foes quite a bit.
Most likely it is to keep the game simple and keep everyone on their initial initiative count, rather than moving it all over the during the course of an action scene. And it can be justified when visualizing the scene too. In the course of combat rounds, you don't have a whole lot of time to plan. You need to choose and act decisively now (ie, as soon as your reflexes let you) or you end up just standing around as the fight goes on around you. You can ready an action to respond to a specific trigger, because you are taking a decisive action to focus on paying attention trying to notice exactly that trigger you are waiting for. If you aren't decisive, you can either just stand there thinking, or you can try to defend yourself while you stand there thinking (Dodge).
However, when you are attempting to surprise, you often want a specific character in the party to act first. RAW, the only way I can see to do that is to either declare the party is attacking and then hope that character wins initiative, or for anyone who beats their initiative to ready an action (and it's iffy whether you can perform a Ready action before the action that initiated combat contact has taken place). Otherwise, when character one says he looses his surprise arrow, everyone has to roll initiative, and those who beat him have to basically forfeit their action until their next turn--which means they might not even get surprise at all.
My solution is to either allow characters who beat the combat initiator to Ready their actions as normal (right after he shoots at the orcs, I...), or to let the character who intends to initiate combat simply act before everyone in this particular round. They act first this round, forfeit acting on the initiative count that they rolled, and then commence acting on their initiative count next turn and for the rest of the battle. I haven't fully decided which one I like best.
Another method would be to allow the person who initiates combat to perform their action for free--essentially before the surprise round, and then act in their normal initiative. The potential problem with this is that they will automatically be able to act twice before their opponents, and potentially three times.
There isn't a delay action in 5e. Not having that action kind of messes up surprising foes quite a bit.
Most likely it is to keep the game simple and keep everyone on their initial initiative count, rather than moving it all over the during the course of an action scene. And it can be justified when visualizing the scene too. In the course of combat rounds, you don't have a whole lot of time to plan. You need to choose and act decisively now (ie, as soon as your reflexes let you) or you end up just standing around as the fight goes on around you. You can ready an action to respond to a specific trigger, because you are taking a decisive action to focus on paying attention trying to notice exactly that trigger you are waiting for. If you aren't decisive, you can either just stand there thinking, or you can try to defend yourself while you stand there thinking (Dodge).
However, when you are attempting to surprise, you often want a specific character in the party to act first. RAW, the only way I can see to do that is to either declare the party is attacking and then hope that character wins initiative, or for anyone who beats their initiative to ready an action (and it's iffy whether you can perform a Ready action before the action that initiated combat contact has taken place). Otherwise, when character one says he looses his surprise arrow, everyone has to roll initiative, and those who beat him have to basically forfeit their action until their next turn--which means they might not even get surprise at all.
My solution is to either allow characters who beat the combat initiator to Ready their actions as normal (right after he shoots at the orcs, I...), or to let the character who intends to initiate combat simply act before everyone in this particular round. They act first this round, forfeit acting on the initiative count that they rolled, and then commence acting on their initiative count next turn and for the rest of the battle. I haven't fully decided which one I like best.
Another method would be to allow the person who initiates combat to perform their action for free--essentially before the surprise round, and then act in their normal initiative. The potential problem with this is that they will automatically be able to act twice before their opponents, and potentially three times.
Is there any rule that you can only have one surprise round? I would think that, as long as one side is unaware of the other, the other side can continue to act with surprise.
In the door-kicking example, it would work like this:
Ernie the Elf is scouting ahead in a dungeon and hears Orcs on the other side of a wooden door. The Orcs do not hear Ernie, who reports back to the party. The party decides they will ambush the Orcs by kicking down the door. As they approach the door, the DM tells them to roll initiative.
Round 1:
-Ernie the Elf has the highest initiative and moves to the right of the door
-Brok the Barbarian gets the second initiative. He moves in front of the door and readies an action to smash it as soon as everyone is in position.
-Kelly the Cleric moves behind Brok and readies an action to aid Brok in smashing the door.
-Womack the Wizard moves a few feet behind everyone else
-They make a group stealth check. Ernie and Brok succeed so the party succeeds.
-Brok the Barbarian smashes down the door
Round 2:
-The DM rules the Orcs are surprised by the door breaking down. The party gets a surprise round as per the rules.
The problem with your example is the that group stealth check needs to be first before the team acts. Moving into position is determined as part of the group stealth check. Declaring actions is done on the turns of the PCs in the first round as per:
Determine surprise, Establish positions, Roll initiative, Take turns.
Surprise is determined before positions are established and before initiative is rolled.
You might be overthinking it a bit. The way I see it, initiative is only rolled (and the resulting turn sequence used) after combat is initated (you can roll it ahead if you like, but it's not used yet). So if the initiating action is the breaking of the door, then that happens before initiative.
I wouldn't change the rules on this. RAW handles it. The PC busting down the door with the lowest init forces the rest of the PCs to use Readied actions. No need to change RAW if RAW doesn't have a flaw.