What was the significance of the bells?
Ramban #1: It is the Kohen Gadol's way of knocking before entering Hashem's domain, and of receiving permission to leave. 1
Ramban #2 (citing the Yerushalmi in Yoma, 1:5) and Rashbam: So that, when the Kohen entered the Kodesh, they would ring 2 and anybody who happened to be present (even angels), would leave, 3 and also when he left. 4
Erchin, 16a: The Me'il atones for Lashon ha'Ra that is spoken publicly. Consequently, 'Let something that makes a noise atone for a (sinful) noise!' 5
Since someone who enters the king's palace unannounced is Chayav Misah, like we find by Achashverosh. See Esther, 4:11 Ramban). See also Rav Chavel's footnotes - as to why this applies the whole year round, and is confined to the Heichal, but not to the Kodesh Kodashim. Refer also to 28:35:2:2.
In order not to intrude on the Kohen Gadol's private audience with Hashem.
So that whoever had left was now permitted to return (Ramban).
What are the implications of the word "Leshareis"?
Oznayim la'Torah: To teach us that the Kohen Gadol is only permitted (obligated) to wear the Me'il when he comes to the Beis-Hamikdash to serve Hashem, but not when he walks in the street
What can we extrapolate from the words "ve'Lo Yamus"?
Rashi: That if the Kohen Gadol enters to do the Avodah minus one of the eight Begadim, he is Chayav Misah. 1
Moshav Zekenim: Al Derech Derush, the verse teaches that one who gives Tzedakah covertly, will not go to Gehinom when he leaves the world.
Targum Yonasan: He is Chayav Misah by a burning fire. 4
This is strange, since the Pasuk is not talking about the Bigdei Kehunah, but about the bells on the Me'il - See Ramban, who also raises other objections to this explanation..
Even if he is wearing the Me'il, but arranges that they do not chime, e.g. he walks a millimeter at a time, or glues the clappers to the bells,,, (PF)
Bi'Yedei Shamayim (Sanhedrin, 83b).
What kind of Misah did Kohen Gadol receive?
Refer to 28:35:2; 1 & 2.
Why was the Kohen Gadol able to enter the Kodesh Kodashim on Yom Kipur without fear, wearing the white Begadim?
Oznayim la'Torah: Because then he entered under the protection of the Shechinah,
Why was a Kohen Hedyot, who did not wear a Me'il with bells, not afraid that an angel would kill him when he entered the Heichal to burn the Ketores?
Oznayim la'Torah: Because he was not on the level of the Kohen Gadol, and the angels were therefore not jealous of him.
QUESTIONS ON RASHI
Rashi writes that if the Kohen Gadol enters missing one of these Begadim, he is Chayav Misah. We already know this, for a Kohen without the Begadim is like a Zar, and a Zar who served is Chayav Misah! And he is liable also for leaving quietly. This is not Avodah! And why did the Torah write this in the middle of the description of the Begadim?
Refer to 28:35:2:2.
Moshav Zekenim (29:9): Our verse obligates a Kohen Gadol who did not wear all eight Begadim. Perhaps only a Kohen without the four Bigdei Kohen Hedyot is considered a Zar.