Crazybrit is right. When someone says a bracha softly, you answer amein the same way. Some people feel like a person who says a loud amein (which CAN be disturbing at times), is “showing off,” as was implied by Wandering Chana. If yuo are doing it to show others what a good little amein sayer you are, you defeat the purpose of doing it for the kovod of Hashem.
Some of the time a loud amein is very appropriate,i.e. when someone comes up to me and says I should be zocheh to marry off my single kids this year. I guarantee you that NO one says amein
louder than I do, in that case. 🙂
I have another issue. A total stranger (frum man)approached my husband and me in a local coffee shop and in a NICE way gave me mussar for saying my bracha over the coffee too softly for HIM to hear to answer amein.