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 
3. To those resonant bells, Chizkuni adds two more notions [#1]: - Focus Time
… 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
The ringing bells were an alarm clock to the har habayit [Temple Mount] community – a reminder that the avodah is in session. And since the kohen gadol was their representative and a messenger can not be better than the community it represents, they ought to pray at the same time of his avodah 
4. Chizkuni # 2 – A Distinguished Look
... so that he shall be recognized and distinguished to be holy of holies from the other kohanim that were working in the mishkan
The kohein gadol was a part and apart. He was from his brethren and yet gadol me’eichav ; he could not be just be one of the guys. Thus the halacha requires that the poor kohen gadol be made wealthy. He also needed to look different. His chiming presence was a constant reminder to those around him of that difference. A Sepharadi vestige may be found in those multi-colored turbans and royal robes of their Chief Rabbi. Even in the more boring Ashkenazi world , Rabbinical garb was always a reality.
5. A fifth sublime approach is added by HaKesav Vekabalah – The Inner Alarm
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
The bells are not for the others; they serve as an inner alarm, an auditory reminder to the Kohen Gadol that he must be utterly reflective of before whom He stands at all times; his outer resplendent glory dare not be clothing-deep. And even when he leaves the mikdash, the mikdash dare not leave him!
For each of these five beautiful notions, one significant detail need be worked out; that same kohein gadol, on the holiest day, the year , wore only white [without the me’il and bells] as he forayed four times into the holiest of places [the holy of holies] !!!
If we work with Rabbeinu Bechayei 1, Chizkuni 1 & 2 and HaKesav Vekabalah, it all works out. Consider:
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.
One approach still confounds; that of Rabbeinu Bechayei’s etiquette. Is Yom Kippur a time to drop formality? With a remarkably disarming comment, Rabbeinu Bechayei.illuminates:
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
A child in desperation does not need to knock on his parent’s door – nor does a spouse who simply needs to talk have to make an appointment. Intimacy allows one to forego formalities. Yom Kippur is the day of shidduchim [cf. end of taanis] and the day of the great rekindling of the romance between Hashem and his kallah, Bnei Yisrael.
And Purim? .... Famously, the Arizal said, it is of the same rarefied nature as Yom Kippur, only a notch higher . In contrast to Yom Kippur where we become white, angelic and then dare to approach the Holy of Holies [without knocking], on Purim, we seek intimacy with Hashem without leaving our world; He comes to us and we find him within the cloak of nature.
Then even before we knock – He will be there.
1. One may also question why the Torah places its stiff death-consequence here - given that whenever one enters the mishkan/mikdash and does avodah, he must wear the requisite clothing. Cf. Rashi, Ramban here for a basic approach.
2. Cf also Rashbam and Rabbeini Behayei for yet another two approaches to this issue.
3. For the correct phrase of dressed to a tea, tee or t – cf. this source http://answers.yahoo.com/question/index?qid=20060722162904AAhrETy
4. Cf. Pesachim 112a and Rashi who quotes Vayikra Rabah 21 that R. Yochanan would also wave or knock before entering his own home
5. In a certain sense, this represents a mini concept of the ma’amados
6.. וסימן הקריאה ונטילת רשות זה היה בכל ימות השנה, וזהו שאמר ונשמע קולו בבאו אל הקדש ולא לפני ולפנים מפני שלפני ולפנים לא היה צריך השמעת קול ולא היה נכנס שם בבגדי זהב אלא בבגדי לבן בלבד, וזו היא מעלתם של ישראל שהכהן הגדול היה נכנס לפני ולפנים ביום הכפורים בלא סימן הקריאה ובלא נטילת רשות