You might want to try the Reverso app, I use it for translating but it pronounces words, forvo does this too.
It's not exactly the reason. The letter 'א can be a vowel, but not necessarily for 'ah' sounds; and usually 'ah' sounds require a vowel only when they come at the end of a word, and then it would usually be 'ה. For example:
Head = ראש (pronounced rosh) - 'א is used for an 'oh' sound
Dove = יונה (yonah) - 'ה is used for an 'ah' sound at the end
Sea = ים (yam) - no letter for the 'ah' sound