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