When I was still living with my wife I started to gradually increase the pitch and modify the resonance of my voice (this was before I told her about my situation). It really was quite gradual, and she honestly didn't notice at all! My male voice is naturally very deep and sonorous - if you know the band Pantera listen to the singer Phil Anselmo talk and that's pretty much my male voice. When I watch home videos from those few months and hear myself speak, it's essentially female, and I think "Oh my god, how come she didn't say ANYTHING about that?". To me the change was very, very obvious, and for her to not even notice was bizarre, in retrospect. We were always pretty open, and she would have said something if she'd really noticed. The only explanation I have is that the change was so gradual that she just became accustomed to it.
But then again, although she is super-intelligent, she is also not the most observant person in the world. I had been on hrt for 8 months or something without her knowledge, and she didn't even notice the breasts or the complete change in skin texture, facial features etc. and believe me when I say those things changed dramatically from hrt.