Yes, you would need to buy OmniFocus for iPhone if you wanted to use it. You get convenient, portable access to all of your tasks. I think you'll wonder why you waited so long to buy it, but if it isn't for you, you can request a full refund from Omni within 30 days of purchase.
I think the other poster is referring to the
Drafts app. I see that it can send text to OmniFocus, but I don't see anything about voice notes, and I'd be willing to bet the price of the Drafts app that it sends to OmniFocus on the iPhone, not to OmniFocus on a Mac.