Is our Hebrew language today different in pronunciation than the old Hebrew?
There are many different ways in which the “Hebrew,” or rather the “Lashon Ha-Kodesh” of today is pronounced. In Israel, the Hebrew dialect is pronounced in a pronunciation that derives mainly, but not entirely, from common Sephardic pronunciation. This is markedly different from the Ashkenazi pronunciation, the Teimani (Yemenite) pronunciation, and other traditions.
Due to the many different traditions, it is impossible to know how the “old Hebrew” was pronounced in biblical and later times, though some traditions’ claim to antiquity would appear stronger than others. The Yemenite tradition, in particular, makes a strong claim to authenticity, based on the ancient nature of the community, and the relative calm it enjoyed throughout centuries of upheaval elsewhere in the Jewish world.
Sources: Many poskim write concerning the legitimacy of different ways of pronouncing Hebrew in prayer, and the consensus is that a person should follow the custom of his fathers. See Iggros Moshe, Orach Chaim 3:5; Minchas Yitzchak 3:9; 4:47; Seridei Eish 2:5; and others.