If you don't make money on ebook, you're unlikely to make money on audio. That being said, if you do, I've found that timing the audio to hit at the same time as the ebook and print book means double the revenue on an audio title. I work hard to release everything simultaneously and I've slowly been building up my audio. My next goal is to get it to 30K a month. It's much slower than building ebooks but I've found it's pretty steady. I would eventually like to get audio to 50K a month but it will take years. It's still a goal I'm working on.