(b) As early as the beginning of the classical period ae ceased to be sounded as a diphthong by the rustics and by the provincials generally. This is expressly stated by Varro in his treatise on the Latin language (iv. 9, and vii. 96 and 97), in which he gives Mesius and hedus as rustic sounds for Maesius and haedus.
(c) This rustic neglect of the first element of the diphthong gradually prevailed until at last ae had only the force of a long e and is very generally so written, e.g. seculum for saeculum, femine for feminae, and even que for quae. But this is as late as the third and fourth centuries A.D. The classical sound was undoubtedly ăé.
AU had the sound of ow in English "now".
(a) Au remained a true diphthong down through the classical period at least in the pronunciation of the educated. The Greeks represent it by αυ, as in Κλαύδιος for Claudius.
(b) In vulgar and provincial circles, au had sometimes the sound of u, the first element of the diphthong being neglected as was the case with ae. Hence we find occasionally in inscriptions such forms as frudavi for fraudavi, cludo for claudo, etc. But the vulgar generally gave to au the sound of ō, as in modern French. Thus, some branches of the Claudian family called themselves Clodii, and we find in provincial inscriptions even at an early period Pola for Paulla, Plotus for Plautus, etc. Suetonius in his life of the Emperor Vespasian tells a story bearing on this, which has been often repeated and is important as showing that even in the Silver Age, au was still pronounced as a diphthong. The anecdote runs as follows: "Having been admonished by one Mestrius Floras, a man of consular rank, that he ought to say 'plaustra' rather than 'plostra,' he greeted Floras the next day as 'Flaurus'"--the point of which is that Flaurus suggests the Greek φλαῦρος, "good for nothing."
EI had the sound of ei in English "feint".
Ei remained a true diphthong in keeping the sound of both its elements; but the combination ei is often found merely as an equivalent for ī. Gorssen remarks that in the root-syllables of the words deiva, leiber, deicere, ceivis; in locative forms; and in the dative and ablative plural of -a stems and -o stems, ei is a true diphthong, but is elsewhere a transition vowel between ī and ē. Cf. Aussprache, I. 719, 788; Ritschl, Opuscula, II. 626; Roby, §§ 267, 268.
EU had (nearly) the sound of eu in English "feud".
Eu remained a true diphthong with more stress upon the second element than upon the first. This is seen by the fact that (rarely) it has passed into ū[1] but never into ē. The combination eu is not often found in Latin except in transliterating Greek words, and in the exclamations heu, heus, and eheu, and in the contractions neu (neve), seu (sive), and neuter (ne + uter). In neutiquam the antepenult is short.
OE had the sound of oi in English "toil" (nearly), or of ōē.