Booger didn't yodel or baroo for almost a year. We tried playing youtube videos of other basenjis, harps, pianos, flutes, etc, but he never paid much attention to those. Then we started playing the "meme" game, which is the most annoying thing ever. You just say "mememememe" over and over in the highest pitch voice you possibly can & then get higher. Eventually we might get a noise out of him. Any and all noise was rewarded with "Good speak!!" and treats.
Then we brought Lola home, and she yodels all the time. When we wake up, when we come home, whenever we're not paying enough attention, etc. When she gets riled up, Booger will start making noise, too, so again, lots of "Good speak!" and treats. Booger has now caught on that the way to get attention when they are both trying to get it is to baroo. So now we sometimes purposely lavish her with attention until he gives us a good baroo & then he gets lots of praise.
Now we can get Booger to make noise by saying "Speak", but the noise varies from a grumble to a baroo depending on his mood.
So to start, try making lots of different annoying repetitive noises at him. That's how we got Booger started. And lots of treats for any and all noise. You can work up to a full baroo later.