I found a couple new ones, closely related to each other.

SIBILANCE as a noun, is a hissing sound a person makes while speaking such as extending the s sound or extending the sh sound or any other hiss sounding noise. Using this word in a sentence, one might say "She used a sibilance when she told her baby 'Hush'."

SIBILANT as an adjective, describes a word that has been pronounced with an extended hissing noise. When used in a sentence, it could be worded "The child actor in the school play used a sibilant noise when he said 'Hiss' when pretending to make the sound of a snake."