Carpet, carpet, carpet... You can get it free from local installers & warehouses that are going to toss their remnants after completing a job. I visited a local Floor Coverings international and was given 2 truck loads of old remnants, which they had been planning on discarding.
You can also visit your local grocer. Ask when the get their dairy shipments and ask if their eggs are delivered with cardboard separators. These come in about 5’ x 5’ sections and look sort of like giant egg crates.
Although this won’t completely ‘sound proof’ your room, it will dampen and improve the sound a great deal and all of the above are free and have worked for our band.