Some other things you might want to ask is: What is the maximum size of the audience (If everybody on the planet that was interested in this topic listened, how many people are we talking about?) Do those people really care about that topic? Are...
Some other things you might want to ask is:
Sometimes we just want to talk to someone else about the subject we love (as our family is tired of hearing me talk about it).
If the goal is to generate income, you might look at the audience and ask, "Do these people have money to spend on my services?" "Can I generate enough downloads to get a sponsor?"
If you can't explain your why, you will never make it through the why.