Just a few random thoughts:
1) If you have the space in the return, the "egg crate" sound asorbing
insulation used in recording studios might make a difference.
2) Also, since there is a garage below you, will your homeowners association
allow you to run 8 x 18 return trunk in the garage to a suitable closet?
Never hurts to ask.
3) Switch to a 95% AFUE furnace - you won't believe how quiet sealed
combustion is. Hopefully the existing venting will allow a double wall pipe,
if not, Tranes furnace is rated for downflow discharge.
4) If your water heater is in the same utility area, can you go to a tankless
water heater and use the space to move the returm
5) change the supply ductwork - add a few supplies sudewall from the utility
area = bigger ductwork = less noise