Thabit bin Ubaid (may Allah have mercy on him) says: I came to a gathering in which Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) was also present. He said: When you greet with Salam, let others hear it, that is, greet with a loud voice, because this is a blessed and pure gift from Allah Almighty.
Benefits and Issues: The one who gives salam should raise his voice to such an extent that the person to whom salam is being given can hear it, and if there are more people present, then he should raise his voice accordingly so that they too can pray for peace for him. However, if in a place some people are resting and some are awake, then salam should be given in such a manner that the awake ones can hear it and the sleep of those who are sleeping is not disturbed.
Source: Fadlullah al-Ahad: Urdu Commentary on al-Adab al-Mufrad, Page: 1005