、。
That guy has no idea he's tone-deaf.
Literal
That-guy, self [subject-が] tone-deaf [quotative-って] thing [object-(omitted)] know-not-[emphatic-んだよ].
音痴 (おんち) literally 'sound-stupid' means tone-deaf, unable to sing in tune — a very common descriptor. By extension it's used for 'X-痴' (e.g. 方向音痴 = directionally challenged, 機械音痴 = bad with machines). The nominalizer こと turns 'he's tone-deaf' into the abstract object of 知らない. ~んだよ is the casual explanatory + assertive sentence ending.