When I was a child I remember asking my mother why she always sang music with words like the ones that are in the title of this post. She responded by saying that it was the way that music sounded.
This always bothered me. There were subtle differences between the way a phrase could sound with one combination of vowels and consonants and they way it would sound with another. The end of a note with an "uh" vowel-type sound, for example, could terminate kind of convincingly with an "m," a "p," a "t," a "b," and not convincingly with an "s" (that hiss would add a bit of percussion to the end of the note), a "k" (because there would be an extra vowel sound after the consonant). We also have a wide variety of end vowel sounds (ah, ee, oo, uh, eh, and o in all their various positions of openness and closed-ness).
Then there are the opening consonants that wind players rely on for actual tonguing technique like "d," "t," "g," "r," and "k," and a whole range of "b," and "p," sounds that suggest percussion. The all-purpose "la la la" is sometimes used exclusively by people who are not deeply connected with the music they are singing. It can also be used as a syllable to suggest a very legato articulation in a soft, sustained lyrical passage. The exclusive choice of "duh duh duh" can suggest the lack of connection to a musical phrase, but sometimes it is the only syllable that will do when trying to imitate the sound of a brass section playing something martial.
Since all notes in a musical phrase are connected to those that come before them and come after them, the regular rules of spoken diction don't apply when singing music with nonsense syllables. The choices we make have everything to do with interpretation. I could sing a phrase with many different combinations of syllables, but ultimately, for each syllabic utterance of an untexted phrase, I must choose only one set of syllables, which I end up doing spontaneously and unselfconsciously. I know that the next time I sing the same phrase it may or may not have the same set of syllables, but I also know that nobody's keeping track.
I wonder if anyone reading this would have the same aural picture of the phrase in the title of the post that I have. I wonder if those syllables will mean the same thing to me tomorrow, or even in an hour.