Many podcasts do not continue with a low volume music bed for their entire podcast, but instead fade the music out after the intro, and perhaps bring it back in for certain segments and the outro section. To fade the music out completely, create a keyframe by clicking where you want the final tail of the fade to end. Then, with the selection tool, click and drag the blue volume line all the way in to the middle point of the wave.
As you get close to the middle point, you will notice that Audacity snaps the fade in more severely, creating a more abrupt stop to the music. Experiment with the different fade curves and see which works best for your material.