Congrats on almost flying!

I was telling another guy that you can get significant noise reduction using
barriers that are not massive.  A good CLOSED CELL FOAM glued to the back of
the firewall will help a LOT and won't weigh much.  Just be sure to keep the
penetrations tightly sealed.

For the rest of the plane, very light weight absorbers like fiberglass
batting or OPEN CELL foam can make a big difference.  Anywere you have space,
put in a chunk of foam.  For exposed areas, contact cement  some fabric over
it.

Good flying!

Dan