I live in a townhouse. Music from the attached townhouse next door can
often be heard very clearly through the cold air return in my master
bedroom, even though it's not loud enough to hear through the walls.
The heating systems of the two homes are not connected. I pulled the
grill off of the cold air return to see if there was any obvious reason
for the sound transmission. I was expecting that all I would see is the
inside of the cold air return duct. What I saw was a large space
underneath my bathtub and off to the left are two vent ducts. One is
the hot exhaust from the furnace and the other is probably the kitchen
fan exhaust. The hot air exhaust is definitely transmitting some sound.
Is there anyway to insulate the exhaust line or the cold air return to
the master bedroom from sound? I'm concerned about creating a fire
hazard by putting material too close to the exhaust line.

Kate