In Basque any A sounds [a] (AH
) and any U sounds [u] (OO). The letter Z indicates a sound like the English S (not like the Spanish S, with is represented by S in Basque, too) and TZ indicates the compound sound of this S with a T. The R is an alveolar flap, just like the simple R between vowels in Spanish (cara, mira).
About the stress, the main stress is over the second syllable and there is a secondary stress over the last syllable. So, more or less: ah-RAHN-tsah-,soo (, represents the secondary stress).