You add trim to the top to fill the gap above the door.
Mount at least one side with some screws so you can remove the trim to get the door off the track.
A stop on the non-pocket side (or even two stops) will helo with noise here.
Resource Conservation Technology (Baltimore, MD) has a number of brush and felt seals for weatherstripping that can also be used to help reduce sound transmission.
Brush seals work best for stopping sound if the opening is as small as possible.
You should be able to get the floor gap down to about 1/16 of an inch.
If the floor is not level add a sole plate and level it to the door.

No matter what you do to a pocket door, the sound transfer through it will be so great that any treatment to the surronding wall would be pointless. Even a hinged door is difficult to seal but at least it's possible if you know how to do it.

Pockets can be sealed, it just takes more work.
Hinged doors are relatively ezsy to sel to at least 50 dB.
I do it all the time for SCIF doors.

brickeyee,

Thanks, I was hoping you'd reply. I'm wondering is there anything I can put between the trim and the door? Basically inside the pocket, behind the trim and up against the door. Brush seals again? And would it be worth it to put MLV on the pocket door itself to help stop sound? What dB do you think you can get pocket doors down to?