Quoted from http://www.italki.com/question/259476 The rule is like this: When following a full morpheme(noun, adjective, adverb, verb etc) start with ‘ㅏ, ㅓ, ㅗ, ㅜ, ㅟ’ vowel after a final consonant(받침), the final consonant changed into representative sound and became a first sound of next syllable. ex) 밭 아래[바다래] 늪 앞[느밥] 젖어미[저더미] 맛없다[마덥따] 겉옷[거돋] 헛웃음[허두슴] 꽃 위[꼬뒤] exceptionally, 맛있다, 멋있다 can be pronounced as [마싣따], [머싣따] 맛있다, 멋있다 should be pronounced as [마딛따], [머딛따] as a rule. But, many Korean people pronounced those as [마싣따], [머싣따] in conversation. People commonly used "맛이 있다" instead of "맛있다". 맛이 있다[마시읻따] can be heard as [마싣따] in conversation. When people say "맛있다", they pronounce it as [마싣따] habitually. So 맛있다, 멋있다 have double standards of pronunciation.

'맛없다' 발음은 '마덥따'에요. '맛있다' 발음은 '마딛따 or 마싣따'랍니다.