Is there an off switch for the equipment?

Not out here; the control room's presumably through the second door on the same wall. 

Unlocked door?

Seems like it.

Should they go ahead and drop the disguises? The only disadvantage to that is the enemy might hear them, since you can't whisper the activation words...

The actual recording booth is should be soundproofed? Let's do it.

Alright.

"Mercury Power, Make Up!" 

"Moon Power, Make Up!"

There's no response from inside, not yet. "Let's go!"

She opens the door and charges through.

There's a woman sitting at the recording desk, something off about her. She's speaking in that same smooth hypnotic voice they first heard over the radio, and she cuts off her sentence and looks up when they come in. "Intruders?" she asks, dangerously. 

"Your evil radio scheme is finished! Moon Tiara Action!"

"Mercury Bubble Spray!"

She bats the tiara aside with a wave of her hand, and calls out, "Dark Power: Black Ice!" Mercury's bubbles of water turn to shards of ice, then fly back at them. Splashes of water on the ground turn to invisible ice, and the temperature in the room starts dropping precipitously. 

At the same moment, the tight veil she'd drawn over her spiritual energy slips through her grasp, and both Sailor Guardians get a glimpse at her power, worlds higher than the monster they faced earlier. 

Ah. That's- not good.

"Watch out!" She tackles Sailor Moon out of the way of a particularly fast-moving shard of ice. 

Shit!

"Thanks. We need a distraction, get her guard down."

"Leave it to me!" She stands, and -

"Mercury Aqua Mist! Shining Aqua Illusion!" Her mist pours out into the room - promptly freezes into shards of ice, fortunately Sailor Moon's still on the ground and out of the danger zone - but it doesn't matter, because when the mist clears there's three Mercuries, none of them where she started, and she's charging for their enemy. "Who are you?!" she shouts. "How dare you steal the energy of girls in love!"

"I am Jadeite." She spins and kicks one of the Mercuries, who dodges - then gets caught by a follow-up slash of ice, shimmering into mist. The woman snarls. "Trash like you don't deserve to know more!" Another Mercury is taken out by a sharp punch - and the third leaps at Jadeite's back. She spins, her back to Sailor Moon, totally focused on her opponents - 

'Sailor Moon! Now! Use Moon Tri-Light Flash!'

Jadeite is vulnerable but so is Mercury can't let her get hurt gotta help!

"MOON TRI-LIGHT FLASH!"

Knowing what to expect now, she aims and fires in the same instant she splits.

Mercury dodges out of the way!

Jadeite half turns, fails to dodge in time - and she takes the hit to her side. It burns through her military jacket, she clutches reflexively at her arm - 

"You'll pay for that." She raises her hand, and her eyes burn, as dark energy gathers around Sailor Moon like icy hands grabbing for her neck -