Hummm... I'm not an expert on this subject matter, but you can take a look at what some individuals use to bass proof and sound proof their vehicles.
At my University, they have a sound proof room, and it's a pretty expensive setup (almost like a chamber) and it's designed to give absolutely no echo no matter how loud you yell or play music.
Try looking outside of the box... I know that some companies and even vacation resorts pay $$$ to pretty much soundproof their generators because they are so loud. It may not be 100%, but does a great job.
There have been already a lot of good examples how to do it more cheaply, but with some hard research, you could find a better way (maybe cheaper, maybe not).
Maybe the cheapest way is to host the party elsewhere lol... or invest in some decent earplugs for sound canceling
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