Great question! In Cantonese (as in Mandarin), all non-sonorant consonants are unvoiced — this includes b, d, g, j, and z. Those letters that represent voiced consonants in other languages are used in Cantonese and Mandarin to represent unvoiced *unaspirated* consonants, as opposed to the unvoiced *aspirated* p, t, k, ch, and c. (Otherwise, we’d need to use diacritics or an H to indicate aspiration.) English speakers don’t necessarily notice this, because English also uses aspiration, and b, d, g, j, and z are not really fully voiced in English (except between vowels or other voiced consonants). Of course, we should try to get it right, but accidentally voicing an unvoiced unaspirated consonant in Chinese almost certainly won’t lead to any misunderstandings. Now, as far as *place* of articulation of z/j/c/ch, if anyone figures that out, please let me know.