The term phonocentrism, identified by the French philosopher Jacques Derrica, is generally defined as the superiority of speech language (e.g. presence) over written language (e.g. absence).
It is also true for the metaphysical superiority of speech language over sign language. The diagram below shows a hierarchial structure of phonocentrism.
Based on the diagram and definition of phonocentrism, below are some real-life examples taken from literature.
"Writing is not language, but merely a way of recording language." 
Writing is invariably regarded as "a substitute of speech", "technology", etc. 
Orality is "natural", writing is "artificial" and text is "dead". (Walter Ong)
"...calls phonetic writing 'true writing' and everything else 'embryowriting'" 
"You cannot hear? How did you learn to write and read English fluently?"
"Speaking is used long before writing is invented."
There are refutable arguments against these assumptions. Philosopher and literary critic Derrida defies influential, phonocentric contributions made by a number of theorists, philosophers and linguists, such as Plato, Aristotle, Saussure, Lévi-Strauss, Rousseau, Walter Ong, Leonard Bloomfield and many others for their implications of phonocentrism in their works.
As some philosophers describe how language reflects human perception of reality, such attitude and perception can be discerned throughout literature of the past two millennia in the following examples of logocentric views in quotes:
sign language is regarded as a "substitute" or alternative of speech language.
In fact, speech language and sign language are independent. Sign language is not a substitute of speech language.
"In some circumstances, indeed, when speech is unavailable whether for environmental, ritual or physiological reasons, gesture can become a form of language all by itself."
Aristotle proclaimed that "Deaf cannot reason without hearing." 
Language is not solely related to speech or hearing as evidence shows in neuroscientific studies that cognitive activities show similar results (e.g. language activities in the left brain) in both manual-visual speakers and vocal-auditory speakers.
"Hearing, of course, is unique among senses in that its medium is also the medium of language." 
Sight is unmistakably the medium of language among visual-manual speakers.
Language is located in the left cerebral hemisphere, where speech is located.
Studies in neuroscience show that language in visual-manual modality also occurs in the left brain.
"You shouldn't speak your language Dene because it is not God's language. You must learn English and French." 
This is another example of linguicism.
 Leonard Bloomfield. Language (New York: Rinehart & Winston, 1993) p. 219 cited in Daniel Chandler, "Biases of the Ear and Eye: Phonocentrism", http://www.aber.ac.uk/media/Documents/litoral/litoral2.html
 David Diringer, one of the authorities on writing system, cited in Elkins, p 127.
 Kendon, Adam. Gesture: Visible Action as Utterance. U.K.: Cambridge University Press. 2004 p.3.
 384-322 B.C. – Aristotle’s philosophy concerning deafness: "Deaf people could not be educated without hearing, people could not learn. ‘Greek was the perfect language; all people who did not speak Greek were considered Barbarians. Deaf equals barbarian.’" – http://www.aslinfo.com/trivial.cfm
 "Words of sense: Chapter 3", p 55. This statement is contradictory, when it comes to sight as the primary medium of language in signed language.
 "Aural Space as Equivalent to Quantum Reality" p 52.
 A personal remark (2008) from Lance Cazon of Edmonton, Dene (Native Indian in the northern regions of Canada), that the priest told him in the 1970s in Northwest Territories, Canada.
Introducing Logocentrism and sign language
Also see signoclasm: slashing hands