That is really naughty, could still be an honest mistake "I told bob to do it, bob did you do it? No, I thought you said Dave was going to do it...' so on and so forth. Still it should have been checked all work done and parts fitted before handing it back to you. I struggle to believe someone working there would put their job at risk for a set of speakers.
All that said, I would be furious as well, but rightly reign it in get your conversation straight in your head and go in controlled and get what you want out of the discussion.