In Greek words written also with z, as Smyrna (also written Zmyrna), it probably had the z sound, and possibly in a few Latin words, as rosa, miser, but this is not certain.
Marius Victorinus thus sets forth the difference between s and x (cs):
[Keil. v. VI. p. 32.] Dehinc duae supremae, s et x, jure junguntur. Nam vicino inter se sonore attracto sibilant rictu, ita tamen si prioris ictus pone dentes excitatus ad medium lenis agitetur, sequentis autem crasso spiritu hispidum sonet, quia per conjunctionem c et s, quarum et locum implet et vim exprimit, ut sensu aurium ducemur, efficitur.
Donatus, according to Pompeius, complains of the Greeks as sounding the s too feebly:
[Keil. v. V. p. 394.] Item s litteram Graeci exiliter ecferunt adeo ut cum dicunt jussit per unum s dicere existimas.
This would indicate that the Romans pronounced the sibilant distinctly,—yet not too emphatically, for Quintilian says, ‘the master of his art (of speaking) will not fondly prolong or dally with his s’:
[Quint. I. xi. 6.] Ne illas quidem circa s litteram delicias hic magister feret.
T is pronounced like the English t pure, except that the tongue should approach the teeth more nearly.
[Pompei. Comm. ad Donat. Keil. v. VI. p. 32.] D autem et t, quibus, ut ita dixerim, vocis vicinitas quaedam est, linguae sublatione ac positione distinguuntur. Nam cum summos atque imos conjunctim dentes suprema sua parte pulsaverit d litteram exprimit. Quotiens autem sublimata partem qua superis dentibus est origo contigerit, t sonore vocis explicabit.
From the same writer we learn that some pronounced the t too heavily, giving it a ‘thick sound’: