Now this guide is nice and all, but I mean is it not easier just to download the audio file and convert it? That is my two cents on the matter... Here is how you do all this:
Excerpt
One method is to use Automator. Open Automator (in the Utilities folder), and create a new workflow. Within Automator, do a search for the "Get Specified Text" action and drag it into the "Workflow" pane (the rightmost pane in the Automator window), and enter "Droid" into the text box for that action.
Next, do a search for "Text to Audio File," and drag that action into the Workflow pane. Change the System Voice drop-down to Cellos, and specify a file name and where you want the file to live. You should end up with something that looks like this
For the full instructions check out the washing Post.