I heard the word もう (meaning "already, not anymore") pronounced as むう in this song: https://www.youtube.com/watch?v=3R3uMTMHgPc at about 1:18, and yet it is transcribed as "mou". Is it a pronunciation mistake or a dialectal pronunciation? Did the person mistranscribe what is said, or did I simply mishear the word? Thanks very much for your answers.
The word actually sounds closer to むう to my ears, and it should definitely be pronounced as モー. I don't think dialects are relevant. In this original video, I can see at least one comment pointing out that part sounds weird.
That said, I don't like to call it a pronunciation (or "input") mistake — although this song is known as one of the most natural, well-"trained" Vocaloid songs by Luka Megurine, her voice is synthesized after all, and her pronunciation is far from perfect. I don't recommend you learn something about pronunciation from Vocaloid songs.