The gemara (Brachoth 13b) records that one is supposed to draw out the ד of "אחד" in the first verse of the Shema:
.תניא: סומכוס אומר: כל המאריך באחד מאריכין לו ימיו ושנותיו אמר רב אחא בר יעקב: ובדלי"ת. אמר רב אשי: ובלבד שלא יחטוף בחי"ת. ר' ירמיה הוה יתיב קמיה דר' [חייא בר אבא], חזייה דהוה מאריך טובא, א"ל: כיון דאמליכתיה למעלה ולמטה ולארבע רוחות השמים תו לא צריכת
There is a teaching: Symmachus says: Whoever prolongs the pronunciation of the word "אחד," his days and years are prolonged for him. Rav Aha b. Jacob said : Especially the letter ד. Rav Ashe said: Only he must not slur over the letter ח. R. Jeremiah was sitting in the presence of R. Hiyya b. Abba, and noticed that he prolonged exceedingly. He said to him : So long as thou hast proclaimed His Kingship above and below and to the four directions of heaven, more than that is not required of thee.
This is not really possible with the way Ashkenazim pronounce the ד.
From a halachik perspective, what are the correct ways to pronounce the ד (dalet)? What are the variant traditions? Is there an academic/linguistic opinion on how it was most likely pronounced from the time of the giving of the Torah to the writing of the Talmud? How old are the 2 variant pronunciations of the ד (with and without a dagesh)? What are the earliest references to this distinction? Is there any halachik significance to pronouncing it one way over the other? Is it just considered a hiddur (beautification of the commandment)?