Whenever I sing on an open "ah" vowel I find it very hard to keep my pitch. I either go flat, sharp, or my voice sort of cracks and my pitch fluctuates. A few examples of words that my voice often cracks on: laugh, cry, back, like, buy. Any words that have that sort of "ah" sound I have this problem with. Is this an issue of vowel modification? Or should I be able to sing these open "ah" vowels without modification? When this happens I am usually not singing very high, maybe A3-D3 (I am a baritone). Is there anything I can do to prevent my voice from cracking on this vowel?