Bran saw this as more of an informal questioning rather than an actual conversation, since he was the one who had lied about basically everything, and of course she would want to separate the lies from the truth. There were questions he wanted to ask her, under the pretense that he didn't know the answers already - like whether or not she could sing - but he figured they could wait. At least until she felt she could trust him enough, although that might never happen.
His real voice was the one Ella had heard, although it was the one he used least often - it was too memorable, and for someone like him, it was much better to be forgettable; that was why the voice he'd used as Trevor sounded so ordinary. "This one. The one I'm using now, I mean." Bran finally managed to stop staring into his mug and looked at her. "I disguised my voice in the catacombs out of... instinct, I guess."