Door messages and noises and memos in scripting?

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http://wiki.frictionalgames.com/hpl2/amn..._functions

1. How do I get a message that says "This door is locked, you need a key to open it" when I interact with a locked door?

For level doors, you can specify the category and entry name in the entity tab when the level door is selected in the level editor.

For swing doors, you're going to need to add an interact callback to the door with SetEntityPlayerInteractCallback() and use conditional statements with GetSwingDoorLocked() to tell whether or not SetMessage() should be used.

2. How do I get the noise when I use a key and unlock the door? Also: How do I get the "solved a puzzle" purple light and noise of accomplishment.

PlaySoundAtEntity()

GiveSanityBoostSmall() or GiveSanityBoost()

3. How can I create a memo to appear after interacting with a locked door? For instance, if I walk up to a door and it's locked I want the memo writing noise to sound and the little diary to light up and a little hint will be written in the journal.

AddQuest()

Another random question: How can I get a rain sound to continually loop in the background?

There are multiple ways, but since you didn't really specify your intentions, i'm going to assume something like hearing the sound of rain when the player is near a window. In which case, placing a sound object that points to the rain sound in the level editor at the window should do the trick.