Animemes

A fairly hard to answer question with Japanese. It operates with morae, not vowels and consonants. な row (なにぬねの, na ni nu ne no) and ま row (まみむめも, ma mi mu me mo) are starting with distinctly different sounds, they are pretty hard to confuse. However, there is also this fucker: ん (n). This one can be read very differently depending on what surrounds it. As an example,

{先生|せんせい} (se n se i), means teacher, has ん usually romanized as "n";

{先輩|せんぱい} (se m pa i), means senior, has ん usually romanized as "m".

There are some more ways of reading it, sometimes it becomes nasal, sometimes it makes you pretend you are speech impaired.

Japanese onomatopoeia for a cat is usually written にゃん (n-ya n). Two n sounds here are a bit different, one is represented by the beginning of に (ni), another by ん (n). The first one is hard to confuse with an "m", so I would say that it's just cats producing a sound somewhere inbetween m and n, and it just so happened that Japanese people attributed it to に.

Happens in plenty other languages, Ukranian one is няв (nyav), for example.

It also sounds similar in most other languages.

They could very well just do みゃ/Mya in their language. Even みゃお/Myao