One thing that I’ve read a few times from Captain’s comments - he has emphasized being able to hear the audio, I believe he said “somewhat.” With people who are deaf, I believe he said that as long as they are playing at a volume a person who possesses the ability to hear could hear it, they would benefit from the field as well. So, I think that if your baby can’t hear it, it will not affect her. The physical fields I would still keep a distance of a few feet away, however I think that listening at a low volume through headphones she would not be able to hear, and you should be fine.
I think the fields designed for environment like powerful positive energy may be designed a little differently, because it has been said with that particular audio that you can play it on low in your bag and it will still work.