It depends on which consonant it is. In this case because of the nasalization rule, consonants that make a 't' sound when they are the final consonant (ㅅ,ㅆ,ㅈ,ㄷ, ㅌ,ㅊ) will change to an 'n' sound (ㄴ) when the first consonant in the next sllyable is ㅁ or ㄴ. So 존댓말 would actually be pronouced as 존댄말.

Also changes to a "t" sound when followed by nothing. For example: 맛

ㅅ changes to a "t" sound when the next syllable starts with a consonant. This page might be helpful: http://www.koreanwikiproject.com/wiki/index.php?title=받침