Is there any difference in pinyin between bia and ba in pronunciation?
They are different. 
ba sounds the same as British "Bar."

bia sounds the same as the last syllable of "Libya."

biaji biaji 的, used to describe the smacking sound of loud eating, e.g. like ducks eating. There is no written character for this word.

有人吃东西为什么嘴老biaji 那么响，让人听着好不舒服. Why do some people always smack their mouths while eating? It makes me very uncomfortable.

Source: http://zhidao.baidu.com/question/27551978.html

Note: Spoken Chinese is much richer than written Chinese. Due to the un-alphabetical nature of the Chinese writing system, it is very difficult to absorb these vivid spoken Chinese words into mainstream writing.