Hi quick question:

Lets say characters are going through a closed door, but had previously heard sounds from the other side. 
On other side of door, a monster awaits.  Monster has also heard characters moving on other side of door and made moves to ambush.  Neither PC nor Monsters have seen each other.

When characters open door, could they be surprised?  Or at that point is the ambush foiled?  I.e. no surprise round for either side?  

Thanks!

Ultimately it's up to the DM, but normally a creature isn't surprised if it becomes aware of a potential threat. If both sides are aware of possible danger beyond the door, you'd need a good reason to say they're surprised.

Quote from callmeLothar >>

Hi quick question:

Lets say characters are going through a closed door, but had previously heard sounds from the other side. 
On other side of door, a monster awaits.  Monster has also heard characters moving on other side of door and made moves to ambush.  Neither PC nor Monsters have seen each other.

When characters open door, could they be surprised?  Or at that point is the ambush foiled?  I.e. no surprise round for either side?  

Thanks!

5e does not have a "suprise round", that's a mechanic of previous editions.

There is a sage advice video about initiative and surprise, that explains everything quite well, if you want to fast forward, the surprise part starts around 6:16.

I suggest you watch the whole video, as it covers a lot of the moving parts to your question.

Thanks for the responses!