There are claims that already in the old kingdom of Egypt everybody used approximation of pi as 22/7 = 3.142857. For sure, it was known at the time of Archimedes. Hebrew alphabet has 22 letters and 7 is the number of the original double letters בגד כפרת according to Sefer Yetzira. In the commentary to the translation by Arieh Kaplan, it states the sound of resh with dagesh was lost or deliberately concealed (1). So pi is alluded by a reciprocal of a fraction of double letters, which themselves hint to rolling/repetition of the alphabet to invoke a second sound.
Alternatively, 7 is the number of days in a week. Incidentally, these are the first two complete quantities defined by the Torah, which brings the question, what can be learned from it?
(1) Quote from Aryeh Kaplan "Sefer Yetzira", Revised edition, Weiser, 1997, p. 160 (the book provides a big list of original sources):
Highly significant is the fact that the Resh (ר) is considered one of the doubles. Most post-Talmudic grammarians take presently the opposite view, and state that the Resh never take a Dagesh. Not only is there no verbal distinction between the hard and soft Resh, but modern Hebrew grammar does not even recognize such a difference in the written form.
There are, however ten different words, appearing in fourteen places in the Bible, which are written with a Resh containing a Dagesh.
See Table 28, containing: 1. haRimah Shmuel A 1:6, 2. haR'item Shmuel A 10:24, 17:25, Melachim B 6:32; 3. Ra Yirmiyahu 39:12, Mishlei 11:21, 20:22; 4. Karat Yechezkel 16:4; 5. Sharekh Yechezkel 16:4; 6. Rosh Habakuk 3:13; 7. LeSharekha Mishlei 3:8; 8. Marat Mishlei 14:10; 9. Rakh Mishlei 15:1; 10. sheRoshi Shir haShirim 5:2.
It is obvious, however that the usual rules applying to letters בגד כפת, do not apply to Resh.
The present sound of Resh is a fricative, and and is therefore most probably the soft sound. The hard Resh was either lost or deliberately concealed after destruction of the Temple. In earlier times, its use was standard, and there is evidence from their transliteration of names, that its pronunciation was known to the authors of the Septuagint (which used double for Sarrah). By the 10th century, however the double Resh was only used by the members of the small Mazya community in Tiberias. Tiberias had been the last city in which the Sanhedrin, the great court which preserved the tradition, had flourished. This was one of the mysteries that the Sanhedrin had entrusted to the community of Tiberias.
According to the Sefer Yetsira 2:3, Resh is in the group of Dentals (זסשרצ). Alon with letters Zayin, Samekh, SHin, Tzadi, it is pronounced with teeth. According to Long Version (of Sefer Yetsira) 2:1, it is sounded "between teeth with the tongue lying down, spread out." We cannot say that it is a rolled /r/ sound, since this involves the tip of the tongue. It would then be closest to the /l/ sound, and should be included among the Linguals (דטלנת). Furthermore, the hard Resh should be a plosive, like all other hard doubles.
There is no /r/ sound in use today that meets these criteria. Furthermore, there is no plosive sound pronounced with the teeth that could be a candidate for the hard Resh. The original pronunciation of this letter therefore remains a mystery.
Some people believe that pronunciation of /ř/ in Czech language as well as /rz/ in Polish language is the original pronunciation of Resh with dagesh.