Śrīla Gopāl Kṛṣṇa Goswāmī shares Prabhupāda's teaching that mental chanting without hearing the mantra doesn't work. Prabhupāda compared it to mental eating: you need to actually do the thing. True chanting requires both speaking and listening to the Hare Kṛṣṇa mantra.
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Swāmi Ji. There was one lady who came to meet Prabhupāda, and Prabhupāda, as usual, preached to her, "Please chant Hare Kṛṣṇa mantra." The lady said, "Swāmi Ji, I'm always chanting." 24 hours. Prabhupāda said, "24 hours? How do you chant?" She said, "I'm mentally chanting 24 hours." So Prabhupāda said, "When you're hungry, do you mentally eat or do you physically eat?" She said, "Physically." Prabhupāda said, "No, you should start eating mentally." So she realized. And Prabhupāda told her, if you want to chanta Hare Kṛṣṇa's Holy Name, You chant it and hear, not just mentally go on chanting. That is as good as no chanting. So this lady was convinced that what Prabhupāda said was right. So Prabhupāda emphasized that we chant the Holy Name and hear the Holy Name. And as the scriptures say, the more attentive you are in chanting, the greater will be the effect And then faster will be the cleansing of the heart.