I use a verbal cue to get their attention before diving into my question, because their ears can hear me. In any case, you need to wait for people to register that they need to change contexts before engaging them.

Where did you find the three dollar discrepancy?

vs.

Hey, Barb. <wait for acknowledgement> Where did you find the three dollar discrepancy?

With earbuds or headphones, the method of attention getting has to change: something they can see (wave), or something they can feel (tap the desk, no touching them.)

Ultimately the point is you get their attention, then you engage with the subject. If you're oblivious to the fact that there's an audio barrier sticking out of their ear, don't be surprised when you get no response.

One can hear, the other can't. So not sure how that's a reverse rule at all, it's actually the same rule applied to the reverse situation. The reverse of the rule would indeed be talking to someone without earbuds in.