If you are using MediaShout 6 for PC or Mac, please see the articles titled the same in their section of the Knowledge Base.
Attached at the bottom of this article is a script with the Audio Cross-Fades setup. You can copy and paste (and adjust) as you need. However, here are the steps to create this from scratch (note: the fades used below are linear and not logarithmic, so that's why the fade in/out times are different - adjust as you need). It looks like a lot of steps, but once you do it the first time, you will see how easy it is.
1) Drag one of your pad MP3 or WAV files from Windows Explorer into your script so that it creates it's own cue.
2) Insert your song lyric immediately after your Audio Cue, so that it creates a lyric cue in your script.
3) From the More Elements Deck, insert a Sound Control Cue immediately after your Song Lyric.
4) Open the Cue Editor for the Audio Cue first.
5) On the General Tab, change the Advance (Out) from Manual to Immediate. This means that when you fire this audio, it will automatically advance to the next Cue (your Song Lyric) automatically and immediately.
6) In the Sounds Tab, check the Fade In box in the top and set to 3 seconds (you may want to adjust this as needed, but be aware that this is a linear fade).
7) Also in the Sounds Tab, uncheck the End with Cue box in the bottom left.
8) Close the Cue Editor.
9) Open the Cue Editor for the Sound Control (after your Lyrics).
10) In the Sound drop down, choose your specific Audio Cue (not All Sounds).
11) In the Action drop down, choose Pause.
12) Check the Box for Fade In & Out.
13) Set the Fade Time to 15.0 (this is in seconds and seems like a long time, but is linear, so it dies out faster than you think - again, you can adjust as you need).
14) Slide the Volume slider all the way to the left.
15) Note, the Advance (Out) is set to Immediate, meaning when you click this sound control, it will immediately roll into the next cue (the next audio file), which will in turn roll into the next Song Lyric. This is what creates your Cross-Fade. Close the Cue Editor.
That's it. Repeat this process for each set of Audio/Lyric set you have. Be aware, that you can easily copy and paste the Sound Control in your script multiple times (so you don't have to do steps 3, 11-14 each time because they are already done...you just need to change the Sound drop down).