What is the interpretation of a sleeping person reciting incomprehensible words in the manner of Quranic chanting, and is this considered a form of polytheism (shirk) or disbelief (kufr)?

What you find of repeating ordinary words or words that have no meaning, there is no harm in it for you and it has no relation to shirk (polytheism). We advise you to fear Allah, turn away from delusions and doubts, occupy yourself with what benefits you in religion and this world, and consistently recite Ayatul Kursi, Surat Al-Ikhlas, and the two Mu'awwidhat (Surat Al-Falaq and Surat An-Nas) before sleeping; for these have great benefit in reciting the words of Allah and providing protection.