Alright, I eventually figured out how to do this using AutoHotKey.
The Alienware x17 R1 doesn't have a play/pause media key like my other Dell does. How can I create one, e.g. by assigning one of the Alienware macro keys to do play/pause (for Spotify / iTunes / etc., depending on what's running at the time)? I know how to go into the Alienware Command Center and create a new macro and assign it to a key, but I don't know what to put into Macros -> Create New Macro - Keystroke -> Enter Keystrokes + Modifiers so that it'll work like the Play/Pause key on my other Dell does?
x17 R1, play pause media keys?