Is it obligatory to re-invoke Allah's mercy upon a sneezer if he did not hear the response, or is the first time sufficient?

If you hear the sneezer praise Allah, then repeat the "tasmeet" (saying "Yarhamukallah") to him so that he hears you, unless he is deaf. The minimum for praising Allah, for the "tasmeet," and for its reply, is for one to raise his voice so that his companion hears him. It is not valid to return the greeting of peace or to say "tasmeet" to the sneezer unless the one offering the greeting or the sneezer hears it, unless he is deaf, in which case one should respond to him by moving their lips.