Styles of singing split into two branches in the twentieth century.
The heavy style of Wagner and Puccini continued throughout the century, tapering off only toward the end. Evidence of this can be found in the concerts hosted by the Richard Tucker Music Foundation which always feature heavy singers. (Curiously, this is this evening and features James Valenti as the 2010 Richard Tucker Award Winner. From San Francisco Leah Crocetto is a grant recipient. She is by far the heaviest woman’s voice among the Adler Fellows.)
Then came Expressionism and Arnold Schoenberg. Here is a bit from Erwartung (1909).
There is a lot of shouting in this short example. Schoenberg is sort of the anti-Wagner. Where Wagner is a sea of tonality with almost constant modulating, Schoenberg carefully avoids establishing a key, and therefore cannot modulate away from it. Wagner is still about singing. His roots are firmly in the operatic world. In Schoenberg we have returned to the world of the monodists. The singing is there solely to support the drama. This is the attitude of all the modernists.
This is a nice bit from Lulu (1979 version) "O Freiheit."
The goal for the singer is to get out of it alive. It may be important to know that Anna Netrebko's manager won't let her sing Lulu, though she would like to.
Modernism requires a narrow piercing sound rather than a large round one. Because established singers are reluctant to perform this repertoire, it can provide an opportunity for young singers. Marilyn Horne made her San Francisco Opera debut as Marie in Wozzeck.
With heavy Wagner/verismo singing the danger lies in over-singing, in pushing the voice to produce beyond its natural capacity. The danger in modern singing lies in never really establishing a proper legato. I must say Marilyn produced the most legato Marie I've ever heard. It is possible to sing Berg and not destroy yourself.
Here is a personal favorite from Nixon in China (1987) "I am the wife of Mao Zedong."
How can you not love this? Kathleen Kim who sang in Tales of Hoffmann is doing it on this season's simulcast.
It is important to know that there is no particular style associated with modern music. Stravinsky for one deeply resented even the suggestion that there was more in the music than was written on the page. Any singer will tell you there is always much much more.