Vowel aperture, roughly speaking, refers to the degree of openness of articulation; In our case, we find two kinds of "o" and two of "e". If we transcribe them the same way, we produce a confusion that never happens orally.
Prosodic features are things like the accent or tones, and because of the semantic values they bear, we should include them in writing. Compare:
In context, the need for distinction is even strengthened:
|mbala moko||mbala mɔ́kɔ́||once|
|mbálá mɔ́kɔ́||one yam|
If we have simultaneously phonetic and prosodic differences, the confusion brought by an "insipid" orthography is still more obvious:
The language also makes use of tones to differentiate tenses in conjugation.
Jacky Maniacky || www.bantu-languages.com/en/characspe.html