Assalaamu alaykum. We know that replying to the greeting of Salaam is obligatory. However, if I reply in a low voice so that the person to whom the reply is given most probably cannot hear the reply, wil I be sinful?

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger. 

Replying to Salaam is an obligation, as you mentioned, and this obligation is not fulfilled until the person who greeted you hears your reply. Al-‘Ayni wrote, “Returning the greetings is not valid unless it is done in an audible voice and the greeting person hears the greeted one returning the greeting, unless he (the greeting person) is deaf; in this case, the greeted person should move his lips so that the greeting person would know that he returned the greeting.” [‘Umdat Al-Qaari Sharh Saheeh Al-Bukhaari]

Ad-Dimyaati wrote, “The least sufficient manner of returning the greeting is to greet the person back in a voice that the greeting person can hear; if he could not hear it, then the greeted person's has not been cleared from his liability regarding the obligation of returning the greeting.” [Haashiyat I‘aanat At-Taalibeen]

As for sinfulness, you bear no sin if you returned the greeting in a low voice that cannot be heard in this situation given the excuse of ignorance; the religious ruling on this matter is unknown to lay-Muslims and this matter falls under the category of religious matters the ignorance of which is excused.

As-Suyooti wrote, “When a Muslim is ignorant of the prohibition of something which most lay-Muslims are aware of, his ignorance is not excused unless he is a new Muslim convert or he grew up in a distant place or in the desert, where people are mostly ignorant of prohibited matters such as adultery, murder, stealing, drinking alcohol, talking during the prayer, and eating during the fast.” [Al-Ashbaah wa An-Nathaa'ir]

Allah knows best.