Hi Thomas.  What's happening in the pronunciation of the phrase "He jumped in" is a result of American English pronunciation and linking.  If a word ends in a consonant sound, and the next word begins in a vowel sound, those two sounds are linked.  Examples:
 
He's angry = He sangry
Keep up = Kee pup
 
 
Also, in American English, when a "t" falls between two vowel sounds, it sounds like /d/.  So, in speaking, the phrase sounds something like this:
 
"He jump din"
 
Another example where you hear a "t" become /d/:
 
Put it away = pu did away
 
 
Hope that helps,
Megan

________________________________

From: englishlanguage-bounces at nifl.gov on behalf of Thomas N. Robb
Sent: Sat 1/13/2007 6:50 PM
To: The Adult English Language Learners Discussion List
Subject: [EnglishLanguage 997] Re: pronunciation of ed


What is happening with the /d/, compared with /t/, endings is technically called "devoicing".  The "voiced consonants" such as /b/, /d/, /g/ and /v/ tend not to be fully voiced at the end of words, so they end up sounding pretty much like their unvoiced counterparts /p/, /t/, /k/, and /f/.  We can't really say that they become the same, however, because if the following word begins with a vowel, then the full voicing is heard.  Compare: 


He jumped.

He jumped in.


Cheers,
  Tom Robb, Japan

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** Thomas Robb, Kyoto Sangyo University, Japan  ** 
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