The most common vowel sound in spoken English is the Schwa sound. The schwa sound is an “uh” sound which occurs in one form or another in all unstressed sylables. In spanish for example all the vowels sounds are vocalised as they sound. The schwa sound is made witha relaxed mouth, jaw and tongue.
In Spanish all of the vowels are stressed, by contrast in spoken English we shorten the the unstressed vowel sounds to the schwa.
Consider the word: “Banana”
/bəˈnɑːnə/
Listen to the difference between a native Spaniard saying the word and a native English speaker.
The native English Speaker says a schwa sound in place of the vowel sound.
banana, button, excellent, colour, sister, picture