No. There no are different in pronunciation, only in grammar. And brazilian people also have difficulty in these case, too. My english isn't good, sorry. Não. Não há diferença na pronuncia, somente na gramática. E o povo brasileiro também têm dificuldade nesses casos.
none, at least in Brazillian Portuguese, however, sometimes the a can have a more closed sound than à, for example in banana, the first a has an ah sound, just like à, a very open sound, but the second one is closer to uh, more closed, similar to the u sound in english like in Bus.