To reduce player's sight range - if the place is closed use line of sight blockers on its borders, if not you could apply negative ultravision- research it when the player enters the zone, remove it when the player leaves the zone.
To slow death anims - You could try the Animation category in the triggers, or slowing the general game speed with the Game category in the triggers... But I'm not sure.
For your informatin the Death time (seconds) modifies the time in between the unit's start of death (in other words, when the unit's hitpoints become 0) till it starts decaying. So no, it wasn't supposed to help.
Sounds are simple: Go to the sound editor and right click the sound you want to play and select the matching from "Use as sound", "Use as music", or set the sound to the value of a variable in Map init or whenever.
In both cases, it will end like this:
-
Events
-

Unit - A unit enters YourRegion
-
Conditions
-
Actions
-

Sound - Play YourSound
-

-------- Note that the sound will be either your variable or the name of the sound that you modified in the Sound Editor --------
Important: You must make the difference between music (in the background) and sounds, like death sounds, monologue sounds and so on. In the sound editor, you are able to edit the properties of your sound and modify it according to your needs.
Good luck exploring WE!