Some Hemi-Sync® products have sections in which the voice is too low for me to understand. Isn’t that subliminal? Or is it a defect in the product?

No to both questions. By the time the voice becomes low in volume, you are in an extremely relaxed state or perhaps asleep. The words you may not understand are simply a repetition of earlier sections of the recording. They are repeated for purposes of reinforcement, at a very low volume so as not to interfere with your state of relaxation.