Assalamualaikum, when it is said that recite surah Bakra so that your houses don't become graveyards, does one have to recite it loudly or by heart is fine too?
Alaikumassalam wa rahmatullah,
The hadith that mentions that (Sahih Muslim 780) is a single-companion hadith that has no supporting hadiths from other companions, so it does not represent an essential part of Islam because single-companion hadiths are inherently doubtful (they do not represent certain knowledge according to the scholars of legal theory). What this means is that the hadith represents merely a recommendation rather than an unchallenged fact. As for whether one should recite it by heart or aloud, the hadith does not clarify it. It uses the word qaraʾa (”to read”, “to recite”), which is used in classical Arabic mostly to mean recitation aloud. But there is no reason to think that only recitation aloud would work for this purpose. We simply do not know with certainty since there is only one hadith on this matter.