> Answering when one's name is called.

> To get attention, when loud voice is needed: [someone far away has just dropped his wallet without noticing] "Yo!!!"

Those are the only uses that would come naturally in my speech, and even the second still sounds creepily newfangled. (When our names were called in grade school in a huge Northeastern city, we still answered "here.")

> To halt an action in progress: "yo yo yo"

Only "Whoa whoa whoa" works here for me. Or "Hold on!" (Note presence of "o" sound in all three choices>)

For some of the others, "Hey!" or "Hey man!" (certainly not "Hey, you!") is the utterance of choice.

In a "quieter setting," it would be "Say, Michael..." Or "Mike" back in the days (I mean "day") when men usually went by the conventional nicknames.

When a car runs over my foot, I say "Owwww!!! &@##@&%%!!!" Not "Yo."

No sentence-final "yo" for me. It would have to be, "OK?" or "man."

And it's just one single-syllable interjection, people! Nobody said language was simple!