Most of the time a rap beat is developed around a very simple melody. Once you can take a melody from your head to your sequencer you are at an instant advantage as a beat maker.
I'm not trying to say that an accomplished player of a real musical instrument isn't valuable, but for making rap beats, it isn't as important. Most established rap music producers just hire real musicians to play anyway. I have found that some basic playing ability is enough to get the job done for 90% of all beat makers.
Playing by ear usually comes to those that have practiced extensively. But there are some programs, books and videos out there to help you develop your ear.
Make no mistake, that it is a skill that can be acquired and like any skill, it will come easier to some than others. And it is a valuable beat maker skill without a doubt.
When making rap beats, be sure dedicate some time to listening to other music and not just enjoying it but listening for the notes and how they fit together. A great beat maker will also be a great fan of the genre as well.
