If correctly pronounced, it should sound like 임니다 (note 임 instead of 입). Saying it like 이미다 is slurring it, and is definitely an incorrect pronunciation. The reason it sounds like 임니다 is because the transition from the P-like sound of 입 to the N sound of 니 can't be made smoothly. The former requires the lips to be closed firmly while the latter must have it open. To pronounce both syllables perfectly, you must give a brief pause after 입, but of course no one makes such an unnatural pause. So the result is the P of 입 sounding like an M which has a similar but more relaxed lip formation than P. Even in other languages, the P-to-N transition can sometimes sound like an M followed by N. For example, if you say "cheapness" fast with some slurring, it can sound like "cheamness". In Korean, this effect is much greater than in English because Korean ending consonants like ㄱ, ㄷ, ㅂ are pronounced "unreleased" as they call it, meaning the ㅂ of 합 does not entail the soft release of the light "pu" at the end as you hear in "hop" in English for example. It just makes the lips close up with hardly a discernable P sound. The effect of such unreleased consonant sounds is that 입 and 잎 are not distinguishable from each other unless there is a vowel sound after them that makes them sound out as the first sound of a syllable (입으로->이브로 vs 잎으로->이프로). If a phrase ends in these syllables, they would sound exactly the same no matter how accurately you pronounce them.

"입니다" sounds like "임니다". not "이미다".