As a native speaker of standard Midwestern American English, "I muhnuh" sounds perfectly ordinary, to me, hearing it from another American English speaker. However, if I knew you were not a native speaker of English, there might be a problem. Since it's very unusual for a non-native speaker to have a command of casual conversation, I might not understand you, or, especially if you didn't say it right, I might think you were making fun of my own pronunciation. The right way to say it is something like [ɑjmə̃ɾ̃ə̃], with reduced vowels (not "uh") and with the n flapped.

As a native speaker of standard Midwestern American English, "I muhnuh" sounds perfectly ordinary, to me, hearing it from another American English speaker. However, if I knew you were not a native speaker of English, there might be a problem. Since it's very unusual for a non-native speaker to have a native-like command of casual conversation, I might not understand you, or, especially if you didn't say it right, I might think you were making fun of my own pronunciation. The right way to say it is something like [ɑjmə̃ɾ̃ə̃], with reduced vowels (not "uh") and with the n flapped.