language is "prototypical" but only in the Eleanor Rosch sense and not in the historical sense. That is, language is central and dominant among socio-semiotic systems but not originary or primeval.I've read the Halliday / & Matthiessen claims before, and to me they read like just what you'd expect linguists to say: to me they come across as axiomatic claims and not conclusions based on empirical evidence.
For me, it's the experience of being a practising — and trained — musician that makes me doubtful about awarding any such primacy to language. When I was learning piano, which I did from ages 8-21 with a number of different teachers, I learned in the appropriate "classical" style which was based on two things: 1. interpreting the notation - i.e. the process of turning the notated score into actual musical performance, and 2. explaining the technique - i.e. the process of what you do with your fingers, hands, arms, (torso, legs etc etc) in order to achieve certain sounds and musical effects. Now the presence of notation in the Western classical tradition means that 1. inevitably involves language — the musical notation is clearly dependent on the prior existence of a writing system for language — the very earliest examples of notation in Europe, devised for the vocal music we now call "Gregorian chant", consisted of "intonation-style graphics" written over the top of the verbal text — and every feature of the notation has its appropriate linguistic label. But not all styles even of Western music depend on notation — perhaps most folk and pop musicians can't "read" — and in the absence of 1., 2. can be carried out largely by demonstration, although the presence of a labelling system, i.e. a technical vocabulary, is very useful for transmission, although not for performance, which in such traditions is largely equivalent to improvising on the basis of fixed "formulae".
Language is set apart, however, as the prototypical semiotic system, on a variety of different grounds: it is the only one that evolved specifically as a semiotic system; it is the one semiotic into which all others can be "translated"; and (the least questionable, in our view) it is the one whereby the human species as a whole, and each individual member of that species, construes experience and constructs a social order. In this last respect, all other semiotic systems are derivative: they have meaning potential only by reference to models of experience, and forms of social relationship, that have already been established in language. It is this that justifies us in taking language as the prototype of systems of meaning. …