Here's why:
* Stress: The word "envelope" is stressed on the first syllable ("en-").
* Unstressed Vowel: The second syllable ("ve-lope") is unstressed, and the vowel sound in this syllable is reduced.
The "e" in "ve-lope" is often pronounced as a schwa sound (ə), which is a very short and unstressed vowel sound. This is common in unstressed syllables in English words.