Many English-speaking people attending concerts sung in English readily state that they cannot understand the words being sung.
In a study, 21 subjects (15 women, 6 men), all Western classically trained performers as well as teachers of classical singing, sang 11 words—“beat, bait, Bob, boat, boot,” representing the most frequently occurring vowels in practice, and “bit, bet, bat, bought, but, book,” representing the other six vowels that occur less frequently—arranged in six random orders, singing on two pitches a musical fifth apart.
The sung words were cropped to isolate the vowels, and listening tapes were created. Two listening groups, four singing teachers and five speech-language pathologists, were asked to identify the vowels intended by the singers. In general, vowel intelligibility was lower with the higher pitch, and vowels sung by the women were less intelligible than those sung by the men.
This according to “Vowel intelligibility in classical singing” by Jean Westerman Gregg and Ronald C. Scherer (Journal of voice XX/2 [June 2006] 198–210; RILM Abstracts 2006-8289).