Need help on volume issue when adding a song from itunes

I am creating a podcast. I am hoping to create a guided meditation CD with speaking and background music. I used the male voice to record the speaking vocals and adjusted some small areas slightly for volume if I was too close to the microphone, etc. It is a twelve minute recording. I then added a music selection (for background) from my itunes by clicking on media browser, audio, and dragging the song into the track area. It automatically put it in Jingles. When I listened to the new recording, speaking vocals and music together, the background music had distinct variations in its sound, going louder and softer on its own, sporadically, jumping in random places. It doesn't do that if I listen to the music selection directly from itunes. So the volume increasing and decreasing only happens when it is brought into garageband. When I hit play on the final piece, the vocals sound exactly as they should but the music jumps around in volume randomly.

I have tried putting the music selection in as a new basic track instead of Jingles.

I have also tried starting with a new podcast, putting the music in first and then adding the completed vocals (I tried it in both male voice and female voice).

I have tried putting the music selection into a project by itself with no vocals and it sounds just fine... the volume problem happens when I add the speaking vocals to the project.

I have also tried dragging the music selection from a file on my computer instead of from itunes. None of these things have helped.

Somehow the vocals and the music together in garageband make the music sound wonky. Please help. Any advice would be helpful.

Thank you

Beau