What is the significance of the bells?
Ramban #1: It is the Kohen Gadol's way of knocking vefore 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 would enter the Kodesh, they would ring 2 and anybody who happened to be present (even angels), would leave, 3 and when they took their leave. 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 Kodshim. Refer also to 28:35:2:2.
In order not to intrude on the Kohen Gadol's private audience with Hashem. 4
So that whoever had left was now permitted to return (Ramban).
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
Ramban: With reference to the bells on the Me'il, the Pasuk is informing Aharon that, if he enters the Heichal unannounced (not wearing the Me'il) he will be Chayav Misah. 2