Do native speakers' ears actually discern the "u" sound in "prueba"?
English is my mother tongue. I can't hear the "u" in prueba. What I hear what I listen as closely as I can to Spanish speakers saying "prueba" is NOT (using American English phonetic spelling here) "proo-ay-bah", all I hear is "pray-bah" as if the word was spelled "preba." In other forms, I can hear it: pruebe and pruebo, I can correctly hear "proo-ay-bay" and "proo-ay-boh." But not with prueba.