Payne states that /p, m/ are often labialized
[pw, mw] and /m, n/ are prenasalized stops before oral
vowels. It is not clear if this means that [mbw] is
normally found. On the assumption that prenasalization is
optional the nasal segments are anayzed as nasal /m, n/.
Yagua has basically a two-level tone system. Other
analyses suggest the mid vowels might be derived from
vowel coalescence, Payne describes these as phonemes with
defective distribution.

Payne, D.L. 1985. Aspects of the Grammar of Yagua: A
Typological Perspective. PhD Dissertation, UCLA.