Sharp eyes caught it all, from the naked panic on Rogers' face that transitioned immediately to clear shock, to the way that Tony's shoulders bunched and his whole body tensed as if bracing for a blow. The former made him feel very smug indeed, because it wasn't as if Jarvis could (or would) reveal his secret to simply anyone, and the novelty of seeing an Avox capable of speech was something applied only to him and never to be recreated. He hadn't spoken to anyone new in years, in fact, and there was something to be said for broadening the carefully insulated bubble in which he survived.
It was Tony's reaction that crushed his glee before it could really take root, leaving him to swallow and glance away from the look on his face. He knew that look. He'd seen it on and off for years, most recently when they'd discussed taking this step at all, and he hated it.
Tony was his world. It was one thing to poke at him when it was only the two of them, to disapprove gently or to roll his eyes in exasperation, but he'd reprimanded Tony in front of a guest... and one Tony didn't like... or trust... or even want to have in the house. Maybe not even in the whole of the Capitol, actually.
Maintaining the bland expression that served him well during public functions, Jarvis sniffed and glanced between the two victors- so very different, and yet he could see where the overlap was becoming problematic. "I believe you're meant to be collaborating," he remarked, a statement that could've been directed at either.
When attention turned to Rogers, however, he took a step closer to Tony; not hiding behind him (impossible, given that Jarvis was the taller of the two), but still clearly more comfortable with the familiar and trusted than this new element that he'd insisted they involve. "It's a voice synthesizer," he explained, that same mechanical hum behind the words. "Tony developed it for my use, though for obvious reasons it has remained a secret. I would greatly appreciate it if you continued that discretion. Please." Plea, rather than threat. Jarvis didn't have the pride not to ask, and he didn't care to add to the tension in the room when his goal had been to dispel it.