How about doing it the easy way (for a change)?
There are any number of technologies using active electronic noise attenuation that can be mounted in ductwork, or even worn as aviation-type headphones that cancel ambient noise while allowing speech and other desirable sounds (such as alarms) to come through. At the very least they could sleep with them on.
Granted, it's not as much fun for NASA as a three billion dollar retrofit would be, but it would work, and an ANR headset typically costs about $1,000 for the very best ones.