This question is about connected speech:
Here is an comprehensive introduction to it from one Keith Taylor, an English teacher:
eslbase_connected speech
A look at assimilation, elision, delayed plosion, catenation and
intrusion in connected speech. In spoken discourse the boundaries
between words are very often not clear-cut. Words and sounds are lost
and linked together in different ways to enable us to articulate with
minimal movement.
This is one of the reasons learners find spoken discourse more
difficult to understand than written discourse. At higher levels it is
often not a lack of vocabulary which prevents understanding, but lack
of ability to deal with these features of connected speech. Native
speakers are more able to use top-down processing to decide whether
what they have heard is red dye or red eye.
Here are some of the more common features of connected speech:
Assimilation
Assimilation occurs when a phoneme (sound) in one word causes a change
in a sound in a neighbouring word. For example, try saying the
following pairs of words:
in Bath
last year
Hyde Park
You’ll notice that the last sound of the
first word changes in each case. The /n/ sound becomes /m/, /t/
becomes /tʃ/ and /d/ becomes /b/.
Elision
Elision is the loss of a phoneme, most commonly the last
phoneme of a word, and most commonly the /t/ and /d/ sounds. Have a
look at these examples:
left back
stand by
looked back
I must go
In each case the last phoneme of the first word is elided (lost). In the most simple terms, the
reason is that the time and effort required to change the mouth
position from the /t/ to the /b/ sound (as in the first example) or
the /t/ to the /g/ sound (as in the last example) is too great!
Delayed plosion Our “red dye” and “red eye” is an example of this. To articulate “red dye”, we must take a very short pause before the
/d/ sound. The /d/ is an example of a plosive, consonant sounds where
the vocal tract stops all airflow. Other examples are /b/,/d/, /g/,
/p/, /t/ and /k/. This pause before the plosive gives us the name of
this feature, delayed plosion.
Another example: the right tie (delay) – the right eye (no delay)
Catenation
In catenation the last consonant of the first word is joined to the
vowel sound at the start of the second word. For example:
pick it up – (learners will hear something like pi ki tup) what is it
– (learners will hear something like wo ti zit)
Intrusion Intrusion is what you might expect from the name – an extra sound “intrudes” into the spoken utternace. Try saying the
following pairs of words:
media event
I always
go away
Do you hear the /r/ sound intruding after
“media”, the /j/ sound intruding after “I” and the /w/ sound intruding
after “go”?
In summary, sounds of words can change in connected speech. The above mechanisms account for most of them.