Aliens can endure a lot more pain than a human can. Usually a different type of screech might help in knowing what emotion it's feeling. In Resurrection it had a monstrous roar when it froze the guy in the room so it probably felt triumph. Maybe the higher-pitched screeches signal pain, who knows.
The aliens tend to screech or scream when in pain, a good example is any of the battle scenes in the movies Aliens. Like Bloo said, different aliens have different thresholds for pain.
lol, smartass.
All I can do is agree with everyone else(Bar alien-drone, we will find out).
Yes, when they scream, you scream, the floor, walls or yourself is melting away, yes, they are in pain.