What is the source of the Ashkenazic practice of the Chazzan saying only certain words out loud? That is, in your typical Artscroll siddur, there are markings for the Chazzan to raise his voice and say a portion of the text.
There is apparently a distinction here between Ashkenazic and Sefardic practice. Thus:
In Ashkenazi tradition, the chazan reads the opening and closing lines of every paragraph of the prayers aloud, reciting the body of the prayers quietly along with everyone else. In Sephardic communities, however, virtually all the prayers are read aloud by the chazan (or a series of volunteers during certain parts of the service).
What is the halachic basis for this practice?
Note: This is related to closed this prior question, which also asked about the repercussions of the practice in that people mumble, and how that impacted her own ability to focus on the words. Commenters assumed therefore that the entire question was not about minhag but about people acting incorrectly. A good answer to this present question might also address people mumbling, either as what is being incorrectly perceived, or as an unfortunate impact. This source and this source might also be helpful in answering the overall question.
