I was told that on Purim when one reads the megillah with the ba’al koreih, only the ba’al koreih's voice should be audible to him and not his own.
Where is the source for this halacha?
Megillas Ester is actually the only exception to the normal rule that you must hear only the voice of the reader (OK, Hallel also, but no one fulfills Hallel by listening nowadays).
תנו רבנן בתורה אחד קורא ואחד מתרגם ובלבד שלא יהא אחד קורא ושנים מתרגמין ובנביא אחד קורא ושנים מתרגמין ובלבד שלא יהו שנים קורין ושנים מתרגמין ובהלל ובמגילה אפילו עשרה קורין ועשרה מתרגמין מאי טעמא כיון דחביבה יהבי דעתייהו ושמעי
In Torah, one reads and one anounces/translates. In Navi, one reads and two can announce/translate. And by Hallel and Megillah, even ten can read and ten can translate. What is the reason? Since it is dear to people, they will pay closer attention.
The Shulchan Aruch O.C. 690:4 says that if you have a non-kosher megillah, you should not read along with the reader, but should pay attention and listen. The Magen Avraham there (:6) implies from the Beis Yosef that the one reading from a non-kosher megillah himself could still hear the Reader, but someone listening might listen to the non-kosher reader instead of the person reading from the kosher megillah. Therefore, you would still fulfill your mitzvah if you read along, but shouldn't do so lest you distract others.
However, if you have a kosher megillah to read along with, then you can read along (S.A. 690:2), at whatever volume you please (although you may get some funny looks if you are too loud).