Learn Italian: Ehi, dove sei? - Hey, where are you?

In Italian, the verb itself tells you who is doing the action. Since sei specifically means 'you are' (for one person), adding the word tu (you) isn't necessary. You only include tu if you really want to emphasize it.

How do you pronounce ehi? Does the 'h' make a sound?

The letter 'h' is completely silent in Italian. You just pronounce the vowels, making ehi sound very similar to the English word 'hey'.

Could I use this exact sentence to ask a stranger where they are?

No, because sei is the informal form of 'you are'. You only use it with friends, family, or people your own age. If you are talking to a stranger or someone you need to show respect to, you would use the formal version, which is Dov'è? or Dove è?.