mytacism

from The Century Dictionary.

• noun A fault of speech or of writing, consisting of a too frequent repetition of the sound of the letter m, either by substituting it for others through defect of' utterance, or by using several words containing it in close conjunction.

from the GNU version of the Collaborative International Dictionary of English.

• noun Too frequent use of the letter m, or of the sound represented by it.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun excessive or incorrect use of the letter M
• noun replacing consonants with an m-sound in speech