No. There is no difference. Now, I assume you know that "baht" and "haht" are not words, but a variation on the spelling only meant to assist in pronunciation. "Ah", which is an expression that varies in meaning (often an exclamation: "Ah!" or "Aaah!".... like "Oh!" in some ways), carries the same sound as the "o" in "not, rot, hot, bot, pot," and the list goes on.... You will also hear this sound in an expression of laughter "Ha ha ha!" or "Hah!" :) Now, there ARE various dialects in American English (specifically with vowels), but the difference is slight and would not matter greatly.