You’re quite right. A lot of people don’t know the huge changes that electrical amplification brought to singing. Singers used to really have to belt things out, so even the back rows of the music halls could hear. With the advent of electric recordings in 1925 and advances in amplification, there was suddenly the ability to experiment and innovate with more subtle singing stylistics. They were breaking new ground all over the place.
Crosby was a big part of that. Along with females singers like Marion Harris and Ruth Etting, and others. A lot of it incorporated the burgeoning jazz styles and improvisation. Some things became quickly dated and were dropped, while other things pointed the way to decades of popular music. But, I suppose like the case with “movie cliches,” which were clever and original when first done, but became so common through the years that they become dismissed and derided, a lot of those earlier vocalists don’t really get the credit I think they are due.
Crosby caught the wave. Microphones and radio.