Use this, you'll need to change the sound path to your sound. You'll need to record or download your own sound though.
Remove the brackets, they're just to help you understand:
:state=0:hudreset,hudx=50,hudy=90,hudimagefine=gamecore\text\pressentertouse.tga,hudname=usedoorprompt,hudhide=1,hudmake=display,state=10 (this just makes the press enter to use hud so it can come up when when requested)
:state=10,plrdistwithin=60:hudshow=usedoorprompt,hudfadeout=usedoorprompt (this means when the player is within 10 units of the door the enter to use hud appears on screen)
:state=10,plrdistwithin=60,plrusingaction=1:sound=x (this means when the player ''uses'' the door the sound will play.)
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