In English, schwa is the most common vowel sound.[5] It is a reduced vowel in many unstressed syllables especially if syllabic consonants are not used. Depending on dialect, it may be written using any of the following letters:
'a', as in about [əˈbaʊt]
'e', as in taken [ˈtʰeɪkən]
'i', as in pencil [ˈpʰɛnsəl]
'o', as in memory [ˈmɛməri]
'u', as in supply [səˈplaɪ]
'y', as in sibyl [ˈsɪbəl]
unwritten, as in rhythm [ˈɹɪðəm]
Schwa is a very short neutral vowel sound, and like all other vowels, its precise quality varies depending on the adjacent consonants. In most varieties of English, schwa occurs almost exclusively in unstressed syllables. (There is also an open-mid central unrounded vowel or "long schwa", represented as ɜː, which occurs in some non-rhotic dialect stressed syllables, as in bird and alert.)
In short, legitimately every vowel is the same sound in English. That's why it always seems like vowels sound the same - it's because they do. American English is the next evolution because they just start chopping out the vowels and pronounce them as if they are separate (sep-ret) from the word.
https://en.wikipedia.org/wiki/Schwa
