His accent, as usual, increased in sibilancy when he was upset.    From Wordnik.com.  [Death of a Scriptwriter]

Certainly Milton would not have avoided them for their sibilancy, he who wrote.    From Wordnik.com.  [Among My Books Second Series] Reference

R, leaves an aperture just so large as to prevent sibilancy, and sonorous air from the larynx be modulated in passing through it; the vowel.    From Wordnik.com.  [Note XV] Reference

L, leaves an aperture just so large as to prevent sibilancy, and sonorous air from the larynx be modulated in passing through it; the letter.    From Wordnik.com.  [Note XV] Reference

W sonisibilant, leave an aperture just so wide as to prevent sibilancy; and sonorous air from the larynx be modulated in passing through it; the letter.    From Wordnik.com.  [Note XV] Reference

The sibilant and sonisibilant letters may be elongated in pronunciation like the vowels; the sibilancy is probably occasioned by the vibrations of the air being slower than those of the lowest musical notes.    From Wordnik.com.  [The Temple of Nature; or, the Origin of Society A Poem, with Philosophical Notes] Reference

The hissing sounds are produced by air forcibly pushed through certain passages of the mouth without being previously rendered sonorous by the larynx; and obtain their sibilancy from their slower vibrations, occasioned by the mucous membrane, which lines those apertures or passages, being less tense than that of the larynx.    From Wordnik.com.  [The Temple of Nature; or, the Origin of Society A Poem, with Philosophical Notes] Reference

If the aperture, made by approximating the back part of the tongue to the uvula and pendulous curtain of the palate, as in forming the sibilant letter H, and the sonisibilant letter Ch Spanish, be enlarged just so much as to prevent sibilancy; and a continued sound produced by the larynx be modulated in passing through it; the letter.    From Wordnik.com.  [Note XV] Reference

Sheila was to learn that the sudden sibilancy of Hamish’s Highland accent meant he was annoyed or upset.    From Wordnik.com.  [Death of a Scriptwriter]