Can minor illusion be cast without line of sight?
For example, could you cast it behind a wall or through a closed door? Or do you need to see where it’s being manifested? I can't seem to find any threads on this.
If a spell requires that you must be able to see the target to cast it, it says so. Minor illusion does not say so:
You create a sound or an image of an object within range that lasts for the duration.
All it requires is that the target area be within range.
However, there is a more general restriction that you have to meet that affects all spells: you must have a clear path to the target (from the PHB section on Targets):
To target something, you must have a clear path to it, so it can't be behind total cover.
And, from the description of Total Cover:
A target with total cover can't be targeted directly by an attack or a spell, although some spells can reach such a target by including it in an area of effect. A target has total cover if it is completely concealed by an obstacle.
So, if the wall or door completely conceals the area you want to target with the spell, you cannot cast minor illusion.