Boris takes out his ear piece in order to hear Sol (I believe) in the scene where Franky Four-Fingers dies. The camera goes to Sol, and when it gets back to Boris, the ear piece is still in. Sure, Boris could have put it back in, but he'd have to be incredibly fast about it.
He doesn't take it out. They aren't even ear plugs. They're hearing aids. Think, the guy deals with guns and ammo, his hearing's probably pretty shot (no pun intended). So he turns it up to listen to Sol, then turns it down just before shooting Frankie.
I thought they were wax earplugs. He put them in before he started shooting, and took the left one out to hear the conversation, if you look carefully, I believe you can see it in his hand.
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