It's hard to tell exactly what you are doing without seeing the code, having said that I imagine that what is happening is that in the function you are attempting to stop the audio source node you might be just creating a new one and that is why it tells you that you can not call stop() without calling start() first.
If that is the problem, what you need to do is keep the reference to the audio source node, so you can access the same one in the function you want to stop it.
Of course, if you can share a snippet of what you are trying to do, it will be much easier to help you.