Sofia is 'The Future' (spoilers)
Could it be possible that Sofia also chose to be cryogenically frozen herself and unfrozen at the same time as David, and it's her voice that he hears as he wakes up in the future? And now he is with her in real life for the first time in 150 years?
Just a thought, similar to "Eternal Sunshine of the Spotless Mind." But I like this idea! I just really want them to end up together :)