dingedheart has excellent advice, both technical and practical. I'd certainly see about changing rooms if that would help.
And I do always find it a bit odd when people feel badly about situations like yours, as if there is some obligation to be comfortable with something you're not comfortable with.
If I had arranged to be home at a time when I could hear a partner have sex, if I was hearing it and it bothered me, I'd wear headphones. I might wear earplugs if it was after bedtime, but I wouldn't want to be required to do that in a live-in situation, but wouldn't mind occasionally.
If I lived with multiple partners and was in your situation, I don't see a single reason at all to ask that if they are going to have sex during sleep hours (I'd probably request specific hours on weeknights, ie 11:30PM - 7am) that they do it as quietly as possible. I deserve my sleep, and sex can still be perfectly fun (even sometimes more fun for some people) if you have to be focusing on being quieter.
Anyway, your poly can be anyway you want, and you can have whatever wants and needs you do, and it sounds like you are very understanding and compassionate. All you can do is reassure OSO that it's not a problem with them. If they choose to be sensitive and decide that you ARE having a problem with them, unless there is some reason why they shouldn't trust you, they have some maturing to do, and some effort should be made to work on communication.
Happiness will never come to those who fail to appreciate what they already have.