Even if he wasn’t wandering around doing it out in the open you wouldn’t Be the asshole. I have my own office and used to absentmindedly sing along to music when I first started working here. I had unfortunately formed the habit at an old job where my office was far away from everyone else and I was going a little stir crazy. One day my office neighbor asked me to stop because it was distracting. I apologizes, was very embarrassed, and “trained” myself not to do it anymore.

Even if you’re an amazing singer (I’m not and this guy isn’t either) it wouldn’t be polite to impose your taste in music on the whole office or risk interfering with their phone calls.

My point is that he might react weirdly because he’s embarrassed (either because he didn’t realize he was bothering people or because he was hoping you were all super impressed with his ahmahzing talent) but that’s on him to deal with.