The good news about the pronunciation of the letter M, is that it is exactly the same as in English and I should now say that there isn’t anything else to say about it, but if you know me at all you will know that cannot possibly be true!
An interesting fact is that Spanish words never end in the letter M, and I have noticed that the Spanish have great difficulty closing their mouths on a consonant at the ends of words. The common consonant ends in Spanish are N, S, R, L, Z, D, J none of which require the closing of lips. That is why names that end in M in English, end in N in Spanish – for example Abraham is Abrahán, Adam is Adán and Miriam is Mirian. This affects the way Spanish people pronounce English, which has a lot of lip-closing at the ends of words. When I teach beginners English classes to Spanish people it’s a devil of a job to get them to say “I’m from …”, as they tend to say “I’ng frong …”