Question

what does the speaker hear in the "deep hearts core"?​

Answer 1

hearts core means deep into the heart

Answer 2

✨"In the deep heart's core" he hears, even while standing on the roadways and pavements of an urban center, the sounds of Nature that fill the air at Innisfree. In his memory and imagination, the speaker hears the quite laughing of small waves along the lakeshore that are a constant sound there.