It is closer to oo-boon-too with the middle oo shortened slightly but that
syllable emphasized. However, you will hear it being said many ways. Oo-bun-too
is often said by North Americans, but people try to correct this pronunciation,
mostly unsuccessfully. I listen to lots of podcastes and the podcasters
themselves have trouble with it and often revert to oo-bun-too, after starting
out correctly with oo-boon-too. Usually it brings laughs when people say it
wrong, but nothing more.