If you're asking about the vowel sound of two kana/mora in the sense of one English vowel sound, you're going about this language the wrong way.
I would strongly suggest that before you continue to study the language, first study the individual sounds of the vowels in Japanese, in the sense of あ い う え お. Once you fully grasp how these vowels sound and can make them individually and accurately, proceed in learning the rest of the kana. These vowel sounds DO NOT CHANGE.
With that in mind, the sound "ai" あい is "a-i", not "ay" like "hay" or "ie" like "pie", however close "ie" like "pie" is.
The important thing to grasp is that "ai" is not one syllable, because Japanese uses a "syllable-like" system called the "mora" もら, which assigns (in the basic sense) one kana, one syllable-length. So, あい is two-mora in length, and so you must consider each kana an individual sound.
Then, as you learn the rest of the kana, you will then begin to group them correctly with this knowledge:
あい is two-mora, "a-i". かい is two-mora, "ka-i". さい is two-mora, "sa-i".
せかい is three-mora, "se-ka-i". きいて is three-mora, "ki-i-te"
...and so on.
As you gain fluency, you'll speak words in a way that doesn't sound disjointed, but rather natural.
Hope this helps clarify.