You can easily get acoustic foam, the type they use for studios. Some is very decorative, so having it covering walls might not seem so obvious. One could always just have some musical instruments or sound equipment sitting nearby as well. Should anyone be in the room and ask, its easy to just say its so you don't disturb the neighbors with your music.

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Come on and let the good times roll,
We're gonna stay here till we soothe our souls,
If it takes all night long.