Shemoneh esrei is to be said quietly. Loud enough to hear oneself but not to be heard by others. Even when davening alone.
If one absolutely needs to daven louder to be able to concentrate, one may daven a bit louder, but only if one is certain that he is not disturbing others.
B’di’eved, if one does not hear oneself, but has uttered the words, he is yotzei.