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I have just begun to study Korean, so I cannot help you. But as additional advice I suggest you forvo (http://www.forvo.com/pronounce/en/) for pronunciation audio. If a word is not there, you can suggest it to be pronunced, after registering and signing to the site.
Since ㅆ as final consonant for some reason is often pronunced as tt/ㄸ, and since 다 itself contains "t" as initial consonant, I could see no difference if the word would be written 웄다 or 웃다.
This is just a guess. Maybe it is an exeption?
Sorry for double-posting. I did not think I could edit my previous reply.