So I've been looking at San Duanmu's The Phonology of Standard Chinese, and in the vowel section, the pinyin /o/ from 中 is not mentioned. An allophone of the middle vowel is described as [o], which is said to appear "in open syllables after labials". The examples given are 我 and 波, so fine we could understand the liprounding in 卓 as being a "bilabial" and so it's realized as [zhwo]. That however doesn't explain 中 which is clearly not an open syllable. What gives?
2 Answers
According to the two-vowel analysis of Standard Mandarin phonology, the statement that:
An allophone of the middle vowel /ə/ is described as [o]
... still 'holds' for closed syllables ending in /-ŋ/, not just for open syllables. The medial can be either /w/ or /ɥ/.
For closed syllables ending in /-n/, the same /ə/ nucleus corresponds to wen / -un (as in 文、敦) and yun / -un (允、群) in Pinyin.
This is actually the 'literal' interpretation of the 注音符號 (aka bopomofo) renditions for Standard Mandarin phonology. See how 征 is ㄓㄥ (zhēng = zh + eng), whereas 中 is ㄓㄨㄥ (zhōng, literally zh + wu + eng) with a kind of 'compound final'. This also applies to the endings in -n (ㄨㄣ for wen = wu + en and ㄩㄣ for yun = yü + en).
start quote << The precise realization of each vowel depends on its phonetic environment. In particular, the vowel /ə/ has two broad allophones [e] and [o] (corresponding respectively to pinyin e and o in most cases). These sounds can be treated as a single underlying phoneme because they are in complementary distribution. The mid vowel phoneme may also be treated as an underspecified vowel, attracting features either from the adjacent sounds or from default rules resulting in /ə/. (Apparent counterexamples are provided by certain interjections, such as [ɔ], [ɛ], [jɔ], and [lɔ], but these are normally treated as special cases operating outside the normal phonemic system.[b] >> end quote
So yes, it is considered a midvowel, but is also underspecified with multiple features/ forms apparent. This is why not everything matches a single format/ IPA value/ whatever you want to call it.
Direct quote from the wikipedia of standard chinese phonology. See the chart of the sounds including o on the original page as well as more info:
