I don't think the jet leg makes her muffled. And Mumei, correct me if I'm wrong, but her resonation needs a bit of work. You can hear it during the Aretha tribute at the Grammys. It's not clear or projected properly, and there could be a variety of habits that can cause that to happen. She's also not very good at enunciating her words, but that's a related issue as well.
Beyonce has good resonation:
Absolutely
no mic or
sound system required, which allows her to
compete with J.Hud when it comes to power, which is
no easy feat.
Christina has volume, but I don't think her ability to resonate properly or project properly is where it should be. I think if you compare her (or, frankly, any of the other girls on that stage) to Yolanda at that Aretha tribute you'll see a good comparison with someone who is resonating and projecting well. I have also talked to royalan about how I think she artificially lowers her larynx more than she should on those lower belts to get that deeper tone (think Charice) and adds extra weight to her voice that her voice isn't made for. She goes for that Whitney-esque sound on those relatively lower (fourth octave - D5) belts. Mariah
used to do it at the beginning of her career, as well, but she was easing up on that by Emotions and it was basically gone by Music Box. I think Christina's vocal weight is probably closer to Mariah, and I think that her attempts to add more weight to it and not coordinate properly hurt her resonance.
And yeah, Beyonce cut right through JHud on that first F5. Beyonce gets some (well-deserved) flack for her use of reverb but if you hear her without a mic she's clearly got the resonance.
Speaking of bad moments, I don't ever recall early Mariah having any major issues while singing, so it was a little surprising when I found this:
Vision of Love
Once in a rare while she had some slight issues. There's a performance of
Without You where she sounded incredible and then something happened with her voice towards the end, at about 2:56. She really eases up after that compared to how she was singing the song before.
She's always had fragile chords, though.