Is one to specifically recite blessings out loud in order so others can answer Amen?
Some Poskim rule one is to specifically recite blessings aloud in order to have others answer Amen after the blessing. One is to try to have at least two people answer Amen after the blessing. If one suspects that the people who hear his blessing will not answer Amen, then he is certainly to avoid saying the blessing aloud.
The Chabad custom: The widespread Chabad custom is to recite blessings quietly. [This custom however only applies to private blessings, such as over a food or a personal Mitzvah, and only when other people may also desire to eat or perform the Mitzvah, and hence they will be saying their own blessing. If however the blessing will not be said by others, and certainly if the blessing is said on behalf of the public such as Birchas Hatorah by an Aliyah, then it is to be said aloud. Likewise, in instances that saying the blessings aloud helps educate the listeners in saying the blessing, then it should be said aloud. Likewise, this custom obviously does not exempt one from saying the words properly and with concentration, and only relates to the audibility of the blessing. However those who say the blessing quietly and quickly, swallowing the words, certainly are not doing the proper thing.]
 Machatzis Hashekel 6/9 based on Asara Mamaors; Zohar Eikev and Vayeitzei; See Shach C.M. 382/4 in name of Mahrashal who writes it is an obligation to recite blessings aloud in order to merit others with Amen and one who does not do so is a Rasha and see Shach ibid that the law that one is obligated to pay 10 Zehuvim for a stolen blessing is only if one could not answer Amen, such as the person who said the blessing said it quietly, and only if one planned to say the blessing aloud; See however Smeh 382/7 and Shach ibid that the above obligation is only by Milah, being it was instituted to be said aloud and in public, however by other Mitzvos, such as Shechita or covering the blood, one may say it quietly and so is the custom; See See Bigdei Yesha 167/33 in name of Kisvei Arizal that if one is alone and unable to have someone answer Amen to his blessing he is to say it with great desire and love of Hashem. This concentration creates an angel who then answers Amen to his blessing.
 Zohar ibid; Asara Mamaros ibid; Machatzis Hashekel ibid
 Ben Ish Chaiy Maasey 14
 Heard from Rav Yehuda Leib and Eliyahu Landa Shlita; The Rebbe by Farbrengens was not heard to say blessings aloud, allowing the public to answer Amen; See also Smeh and Shach ibid that only by Milah is it an actual obligation
The reason: As Admur 213/4 rules that one who hears a blessing according to some opinions is Yotzei even if he did not have intent to be Yotzei, and hence in order to prevent others from entering into this doubt we therefore recite the blessings silently.