When the leader gets to just before the Shacharis Amida, how should he say the blessing
go-al Yisroel: aloud or saying the word Yisroel so the end of it cannot be heard?
The Shulchan Aruch in Orach Chaim 111:1 states that there should be no Hefsek between Geula and Tefila. Although the Rama says there is no problem answering Amen the accepted custom is that we do not answer Amen after Go'al Yisroel. Both options mentioned are options that are acceptable. Some Shuls the Chazan just says the last word quitely and in some Shuls the Chazan says it out loud and the people say it along with the Chazan. The Chazan should do whatever the Minhag of the Shul he is Davening in does.
There is a long (20 pages) and detailed discussion of this question in the sefer Iyunei Halachos by D. Y Zvi Rabinowitz published in 2003. It includes all the sources that discuss this question and the opinions of the recent poskim. While there is no conclusive answer because there are opinions on both sides of the question, it is certainly worthwhile to read to find out what the issues are.
The minhag at my Sephardic community is to say Ga'al Yisra'el out loud, then say "Ad-nai" (as part of "... sefatai tiftah...") in an audible undertone so that the members of the tzibbur would start their amidah right away.