The Rambam (MT Brachot 1:7) writes you should ideally hear yourself
A person should recite all the blessings loud enough for him to hear
what he is saying. Nevertheless, a person who does not recite a
blessing out loud fulfills his obligation, whether he verbalizes the
blessing or merely recites it in his heart.
However R Binyomin Forst in his Laws of B'rachos writes
Most authorities reject the view that one can mentally recite a
bracha and fulfill one's obligation (SA OC 206:3). A bracha must be uttered by the movements of one's lips pronouncing each word
(but R Shlomo Zalman Auerbach allows lips pronunciation even if no
sound is emitted).
There is considerable disagreement in the Mishna and Talmud (Brachot 15a and Megila 19b) as to
whether it is necessary for the bracha to be audible to the person
making the bracha. Though it is clearly desirable that the bracha
be recited in audible voice, the failure to do so does not invalidate
the bracha (SA OC 206:3)
As you see these sources do not mention that others need to hear you. Personally I would imagine one rationale for doing so is to allow others to say amen to your bracha.