Sayyidi wa sanadi Mufti Mohammed Taqi Usmani (Allah preserve & protect him) is asked regarding the recitation of the Qurʾān at graves, whether it should be audible or not. The Salām should be audible and the Qurʾān should be read silently, he says.

I ask Mufti Ṣāḥib regarding conveying Salām at graves on behalf of other people. He answers that this practice is established for the Prophet ﷺ, and suggests that perhaps qiyas can be done upon this for others. 

Source: Seven Days in Bukhara and Samarqand