What's proper greeting protocol?

If you kind of softly greet someone as you pass, then they say hi to you, but you’re not sure if they actually heard you ‘hi’, should you repeat? Risks are they have heard, and will think you’re weird; but if they didn’t hear, they’ll think you’re rude. Oddly, this seems to come up a lot in my life… I should probably start speaking louder in the first place! Meanwhile, what to do?