I'm in a similar situation, my 4 year old son won't let anybody in our home sing, whistle or even dance at times. If you start up when, say, doing some chores, he will scream and scream and hit you, kick you, and smash his head into the floors, walls, throw items etc until the offender stops.
We've actually been pretty strict and consistent about it I'd say, although the singer/whistler is usually put off as it sucks all the joy out of it!
I used to be a good singer but since I can't practise and spend more time shouting, my voice is broken now and I don't bother practicing guitar anymore as I can't sing with it.
There's times he's tolerated some singing, but I can't spot a pattern as such. It's hard, because singing is used a lot for teaching at this age. I wonder how he will behave at school!
To be fair he is autistic, but I've never known an autistic child to act quite like this, it doesn't seem like a meltdown to me ( known a lotta autistic people and kids, and am autistic myself)
I've been using a site called "hand in hand", and it's helped a lot with our bond and his behavior in general, but this one issue persists like no other. It's been going on for 2.5 years.
Watching in solidarity!