Is whispering or secretly saluting the dead from inside the car sufficient when passing by graves?

The hadiths regarding the Prophet's (peace be upon him) salutation to the dead indicate that the greeting should be audible, as in the hadith narrated by Ibn Abbas (may Allah be pleased with them both): "Peace be upon you, O dwellers of the graves. May Allah forgive us and you." It appears that the sufficient manner of greeting the dead is the same as that for the living, because the Prophet (peace be upon him) addressed the dead as he addressed the living, and informed us that they are equal in hearing, as he said concerning the slain at Badr: "You are not more hearing than them, but they do not reply." And what is sufficient for greeting the living is to raise one's voice with it so that it is heard. If it is said that the slain at Badr were a special case, then it might be permissible for the greeting to the dead to be in a whisper, and the Prophet's (peace be upon him) audible greeting to them would then be for the purpose of teaching or because it is more complete.