Ah yes, bring Kaufman, Murray, Burr, and the old-guard singers up to the bar once again. Guilty, your honour, our singing was good enough in 1910, 1915, and 1920, but we are not the best choices to render a vocal chorus on these jazz records. We can each modulate our voices to the demands of the microphone, which is the equivalent of teaching an old dog a new trick, but our singing styles have become passe indeed. Please note that we had greater longevity in our time than the ersatz crooners of the late 20's: Baur, Ellis, Lambert, Downey, James et al who will be tidal-waved out in 1931 by Bing Crosby.
I say all of the above tongue-in-cheek. I think it amusing that there exists such a critique by 20's jazz lovers of vocal styles of the musical era that had just ended.
More substantively, I don't think Trumbauer's voice would have done very well on an acoustic recording. He doesn't project enough, and that odd timbre to his voice is an acquired taste as it is: its delightful laziness just wouldn't do well with a recording horn. It's better that he waited until later in 1928 to vocalize.