The reason why G-d created speech was in order to serve Him, by studying Torah, praying and so on. If one uses the gift of speech for idle talk, one is effectively stealing from the Sitra DiQdusha (side of holiness).
The Gemara of Baba Qamma speaks about two areas where the laws pertaining to stealing do not apply, unless it is in a place where people are strict about it. This can also be applied to two areas of permitted speech which are not specifically for the service of G-d. They are:
1. Speech pertaining to one's livelihood and health. The other area is
2. Derekh Eres, where common courtesy dictates that one must speak.
In a place where one is strict not to talk about these matters, however, speaking about them would be considered stealing.
A Beth Midrash (House of Learning) is a place where people are strict, since it steals time for Torah study away from people who are learning. It says in the Gemara of Berakhoth that Beth Shammai is of the opinion that Habdalah should not be recited loudly at the conclusion of Shabbath, because answering Amen will distract people from their studies. For the same reason, one should not say "Bless You" when someone sneezes in the Beth Midrash.
(See Baba Qamma 119a. Berakhoth 53a. Ben Yehoyada)
Bava Kama, Baba Qamma, Mas. Brochos, Berakhoth, sneezing in synagogue