As discussed under Alma 32:28, the original text here in Alma 34:4 had the singular heart, which is what Oliver Cowdery wrote down in 𝓞 and later copied into 𝓟. At that later time, he consciously decided to pluralize heart: he inserted inline the plural s in 𝓞 and 𝓟, the s in both cases with heavier ink flow and very awkwardly. Usage elsewhere shows that the singular is probably the correct reading here.