Which vowel sound is different in these words?

The answer is apple.

"A vowel" is a sound pronounced with an open vocal tract, so that the tongue does not touch the lips, teeth, or roof of the mouth, such as the English "ah" /ɑː/ or "oh" /oʊ/. There is no build-up of air pressure at any point above the glottis.The word vowel comes from the Latin word vocalis, meaning "vocal" ("relating to voice"). In English, the word vowel is commonly used to mean both vowel sounds and the written symbols that represent them.The name "vowel" is often used for the symbols that represent vowel sounds in a language's writing system, particularly if the language uses an alphabet. In writing systems based on the Latin alphabet, the letters A, E, I, O, U, Y and sometimes others can all be used to represent vowels. However, not all of these letters represent vowels in all languages, or even consistently within one language (some of them, especially W and Y, are also used to represent approximants). Moreover, a vowel might be represented by a letter usually reserved for consonants, or a combination of letters, particularly where one letter represents several sounds at once, or vice versa; examples from English include igh in "thigh" and x in "x-ray". In addition, extensions of the Latin alphabet have such independent vowel letters as Ä, Ö, Ü, Å, Æ, and Ø.