At normal speed, the action of the folds is too fast to observe. Play the first movie and you’ll see that the cords are held loser together. You may get a sense that the folds are vibrating, but you wont be able to see their action in any detail.
Now play the second movie to see the vocal folds switch between voiceless mode and voiced mode, demonstrated by alternation between voiceless [s] and voiced [z].
To see the action of the folds in voiced mode, we need to slow them down using stroboscopy. For this we’ll switch to a recording of the vowel [i]. First, play [i] at normal speed.
Now play [i] ‘slowed down’. You’ll be able to hear that the audio is in fact at normal speed, but the stroboscope creates the allusion of slow motion. Viewed this way, you can see that the folds have a lovely longitudinal rippling action, where the opening appears firstly at the arytenoid end, and works its way along the length of the folds, and then closes again working back towards the arytenoid end.
All languages have speech sounds produced in Voiced mode. English sounds produced this way include; [b] in box, [d] in dog, [g] in go, [v] in vat, [ð] in them, and [z] in zoo.
It is normal for vowels, nasals, laterals and semivowels to be voiced in all languages, although some languages have voiceless versions in addition to the voiced ones.