Your best bet is to carry on watching things at normal volume... otherwise the baby will wake up all the time at the slightest noise. I've got an 18 month old son who sleeps through anything now


Your AV amp will have things like a night mode to emphasise dialogue instead of the loud noises... so just enable that instead.
My wife's cousin had a baby at roughly the same time and they did the quiet approach... and now their baby wakes up whenever either of them so much as uses the toilet at night
Headphones wise though you can't go wrong with a couple pairs of Sennheiser RS-140s or work back down through that range as price suits your needs.