I don't have kids but I'd imagine you wouldn't want to be separated from any noises and such, right? Playing with lower TV volume is a no go?
I use headphones that don't completely cover my ears, so I can hear baby noises.
The main problem with using the TV sound is that the TV with the Wii U is on the other side of the house and too far to hear if the baby is awake (at least, with the game playing). My ideal plan would be to move the Wii U closer to the bedroom, or even in the bedroom, and use the Gamepad with headphones, but I can't do that.
It's hard to explain all the little details of how the house is laid out and how we handle the baby at night. I'm going to try and fix the sound issue tonight and just hope I don't break the Gamepad any further.