The Kohen Gadol dons his eight special garments, resplendent with glory and laden with meanings deep and deeper. Unique in the crowd is the meil, - the techelet bleu overcoat with is hem appendage that sparks great wonder: [Shemos, 25:33-34]
And on its bottom hem you shall make pomegranates of blue, purple, and crimson wool, on its bottom hem all around, and golden bells in their midst all around A golden bell and a pomegranate, a golden bell and a pomegranate, on the bottom hem of the robe, all around
Why the bells and [not whistles – but cloth] pomegranates? For now, we deal with the bells. The text elucidates - with a consequence
It shall be on Aaron when he performs the service, and its (his?) sound ונשמע קולו shall be heard when he enters the Holy before the Lord and when he leaves, so that he will not die.
First note that the text is ambiguous: Whose sound can be heard - the kohein gadol’s voice or the sound of those bells? More significantly, as we try to probe the Divine mind, what is the tachlis – the purpose of these bells?
On our 2nd question, herein five fascinating approaches:
1. Rabbeinu Bechayei #1 - Dressed to the halachic T 
Aharon’s voice - for when he enters the kodesh with those 8 garments in the appropriate manner, his voice shall be heard and his prayer accepted
Aharon’s voice can certainly always be heard – but nishma in the sense of acceptance [think na’aseh v’nishma] A pre-requisite for the Kohen Gadol’s prayer to be heard and accepted is the donning of his proper priestly garb. [On this approach, Ramban and others question why the Torah hinges the prayer acceptance davka on the bells?]
2. Rabbeinu Bechaye #2 – A lesson in Divine etiquette
The sound of the bells must be heard before Aharon enters, for this is appropriate conduct… the Torah is thus teaching that that one who enters before the King must knock at the entrance … and not enter suddenly. The kingdom of heaven is like the earthly kingdom
One does not barge in on the king or The King. A beautiful idea, with a fascinating Purim connection:
.. just as it says regarding Achasveirosh – that the one that was not called and arrive suddenly shall be killed [Esther, 4:
Sans the mishkan/mikdash two vestiges of this halacha still remain
a. The mah tovu that announces our presence before we walk into the synagogue
b. the minhag to knock on the door of any room before entering 
… so that the people would know the time when the avodah [Temple service] was being done and would focus their kavanah to their Father in heaven
... so that he shall be recognized and distinguished to be holy of holies from the other kohanim that were working in the mishkan
the mitzvah of the bells is like that of tzitzis; there the mitzvah is to remind one of the Mitzvos through the sense of vision [and you shall see them] and here it is to remember through the sense of hearing – for commensurate to the kohen gadol’s exalted level and his obligation in many more mitzvos than the rest of the [kohanim and the Jewish] people, there was added an additional [sensory] reminder … through his hearing … the bells he will awaken his mind and heart to consider before Whom he is wearing these special garments … and when he leaves … the sound of those bells will remind him that he is the chosen of the nation … and he will not forget that he is constantly standing before Hashem, nor should he lose focus from his holiness for but a moment
a. Dressed to the Halachic T: – Since the requirement was only 4 garments, then the he certainly fulfilled it
b. Distinguished Look : - On Yom Kippur, the Kohen Gadol was the only one doing the avodah: thus he stood out שהרי כל עבודות אינן כשרות אלא בו.
c. Focus Time – On Yom Kippur, the crowd [hopefully] need not be told to focus
d. The inner Alarm – On Yom Kippur, the Kohen Gadol is highly reflective and highly aware anyway.
And the sign of his entering was for all year round in the holy – but in the holy og holies it did not need a sounding of the voice … and this is the greatness of the Jewish people that the kohein gadol would enter deep within on Yom Kippur without a sign and without permission