That may be going a bit far. Certainly people singing spontaneously by themselves or in groups was more normal and accepted in the past, but no one actually sang a song of their own spontaneous invention concerning their personal feelings about the moment while riding on a crowded bus while others looked on without the least surprise, in real life.

The truth is somewhere between “spontaneous singing was normal” and the fact that we grew up with the musical comedy as an accepted genre, and so the required suspension of disbelief passed without thought. It’s not as great a suspension as it would be for a someone born in the last three decades who has no concept of non-professionals spontaneously singing, but it is not because the kind and occasion of singing we see portrayed in musicals was entirely normal, either.