The name of the company is the Korean word for "Modernity". It is pronounced like "Yun" with an "H" in front of it. Or "Hun", but with the "Y" from "Yum" after the H.

"He Yawn Day" is pretty close, but slur the first two words together into a single syllable, and make it rhyme with "Sunday"

The wikipedia page for Hyundai has a recording of the correct pronunciation.