I'm trying to play a soundfile (named after spellId) using the trigger: Spell / On Cooldown Ready Event + Actions "On Show" Custom Code.

local spellId = aura_env.states[1].spellId
However, this returns nil, which means I can't use it like this...
"... \\Sounds\\%s.ogg"

Please take a look at adding this functionality. Or if the functionality exists, help me see.

The contents of the state depend on the trigger's needs for conditions and text replacements, not on what arbitary custom code wants.

I this case I don't see a reason to add it, since you can easily accomplish your task by either just playing the sound via the built-in methods, or modifying your custom code to run the correct sound.