Everyone agrees that Tav with a dagesh is pronounced as the letter “T” as in “tea” or “too”.
Many Sefardim really do not distinguish between Tav with a dagesh and Tav without a dagesh. Some simply “stress” or “emphasize” (almost like a double tt) when it comes to Tav with a dagesh.
Theoretically, it would appear then that the original Tav sound (without dagesh) is the “th” as in “think” or “Thursday”, which I believe some Yemenites possess as their tradition.
The other “th” sound as in “this” or “that” would therefore be the Dalet sound without dagesh. That being true, then the last word of Shema (Echad) would sound more like “Echa<th>”, the “th” being elongated a bit.
The Tav (without dagesh) being pronounced as a “s” is indigenous to the many Ashkenazi Yeshivish and Chassidic (or Hassidic) Kehillot. While poskim maintain that everyone has to keep to their traditional pronunciations, it appears to me that “s” sound for Tav (without dagesh) is a violation of linguistic rules since “t” comes from one area of the mouth (D.T.L.N.T.) and “s” from another (Z.S.SH.R.TZ).
Another interesting anomaly which is widespread among all Ashkenazim and the vast majority of Sephardim is the pronunciation of “v” as is in “value” for Vav, when it might have originally sounded like “w” as in water. The theory behind this change is that all Ashkenazim and the original Sephardim were influenced by European dialects, almost eliminating the “w” sound, whereas again in Yemen and isolated areas in Arabia, the “w” sound was maintained. Note that the Arabic language has a “w” sound for its 6th letter.