Does burping aloud during prayer—if two letters are produced—invalidate the prayer, or is it not considered speech?

Belching during prayer does not invalidate it unless it is an act of playfulness. It appears that the questioner belched out of ignorance, believing it would preserve her ablution, and not out of playfulness. Therefore, her prayer is valid and she does not need to repeat it. Whoever commits an act that invalidates prayer out of ignorance does not need to repeat it.