Here's how it works:
* Schwa is represented by the symbol ə and is a short, unstressed vowel sound. It's the most common vowel sound in English.
* In "mystery", the first syllable "mys" has a schwa sound because the stress is on the second syllable, "te".
* The "y" in "mys" is not pronounced as a full vowel sound, but rather as a consonant sound similar to the "y" in "yes".
So, the sound of "mys" in "mystery" is essentially just the schwa sound: ə