Srila Prabhupada stresses the importance of Sravanam in several places. From http://www.prabhupada.org.uk/sp25.htm -
The bhakti-yoga process is practised by the devotees in different methods like hearing, chanting, remembering, serving the lotus feet of the Lord, worshipping, praying, rendering service in love, becoming friendly, and offering all that one may possess. All nine methods are bona fide methods, and either all of them, some of them or even one of them can bring about the desired result for the sincere devotee. But out of all the nine different methods, the first one, namely hearing, is the most important function in the process of bhakti-yoga. Without hearing sufficiently and properly, no one can make any progress by any of the methods of practice. SB 2.2.36
I want to know if reading Srila Prabhupada's books is also carrying out Sravanam. Are there any rules such as one has to read loudly and hear the vibration, or will mental sound suffice?