Could someone link to a youtube video or sound file?
Is this a trick question? Glottal stops are voiceless. It's the explosion of sound afterward one can hear.
Say cac-tus for a consonantal beginning or end with a glottal stop. Or gag.
Say Ohio for an initial vowel, and you will probably start with a glottal stop. On the other hand, in singing, it would not be necessary to use a glottal stop on the initial vowel (and I certainly avoid them as much as possible, as I don't like them vocally!).
The reason I ask is that I have seen Renée fleming being criticised for her overuse of glottal stops - as if it was an affectation - but I can't really work out was is meant here.
Perhaps the Wikipedia on glottal stops with the soundbyte may be of some use in sorting this out:
http://en.wikipedia.org/wiki/Glottal_stop
Perhaps it is that sort of "force" that occurs with glottal stops that Renée Fleming is criticized for? I personally have not noticed it in her singing, but, then again, I am a pianist, not a vocalist.
I'm wondering if there is some confusion here between "glottal stop" and "coup de glotte" (stroke, or shock of the glottis): http://en.wikipedia.org/wiki/Coup_de_glotte