Seconding this, also, reproducing her voice with an AI may not be something she is on board with, it could make her feel like you don't accept her with or without a voice. It may also be unhealthy for you, similar to how spending too long on social media can become a dangerous source of dopamine.
It might make sense to consider making a recording that is more meaningful, and focus on giving her emotional support rather than building an AI that could be perceived as a replacement.
It's not like OP is replacing her entirity with Alexa, if I were the wife I'd think "sure, let's 'backup' my voice, having it available in case I lose mine would be useful, so that people can still hear my thoughts in my voice instead of a robot's."...
> if I were the wife I'd think "sure, let's 'backup' my voice"
That very well seems to be the OP's position as well. That's a far more generous reading of the situation. It makes sense that someone here would have the mindset of "lets keep a backup in case we want access to it later."
It might make sense to consider making a recording that is more meaningful, and focus on giving her emotional support rather than building an AI that could be perceived as a replacement.