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.
Looks like a perfectly normal "Enter" key on an ANSI keyboard. Why the confusion?