X suffers a long vowel before it, being composed of the c (the only mute that allows a long vowel before it) and the s.
Z probably had a sound akin to ds in English. After giving the sound of x as cs, Marius Victorinus goes on to speak of z thus:
[Keil. v. VI. p. 5.] Sic et z, si modo latino sermoni necessaria esset, per d et s litteras faceremus.
[ Quantity.]
A syllable in Latin may consist of from one to six letters, as a, ab, ars, Mars, stans, stirps.
In dividing into syllables, a consonant between two vowels belongs to the vowel following it. When there are two consonants, the first goes with the vowel before, the second with the vowel after, unless the consonants form such a combination as may stand at the beginning of a word (Latin or Greek), that is, as may be uttered with a single impulse, as one letter; in which case they go, as one, with the vowel following. An apparent exception is made in the case of compound words. These are divided into their component parts when these parts remain intact.
On these points Priscian says:
Si antecedens syllaba terminat in consonantem necesse est et sequentem a consonante incipere; ut artus, ille, arduus; nisi fit compositum: ut abeo, adeo, pereo.
Nam in simplicibus dictionibus necesse est s et c ejusdem esse syllabae, ut pascua, luscus.
M quoque, vel p, vel t, in simplicibus dictionibus, si antecedat s, ejusdem est syllabae, ut cosmos, perspirare, testis.