Hi, Creative_Casey,
So, as I understand it you are trying to place one audio track under another audio track like this:

In that case, both tracks should play simultaneously unless you mute one or the other. Check to see that the lower track is not muted, either from having clicked on the mute button or from dragging the horizontal volume adjustment line to the bottom. Unmute if necessary, so that it shows wave forms. Also, it might be that the lower clip is DRM (copyright management) protected, in which case it would show no waveforms and would not play. Does the lower sound clip play when it is moved to a side by side position with the other clip, instead of below it?
If the above doesn't solve it, try deleting preferences. To delete preferences open iMovie while holding down the Option and Command keys and selecting to delete preferences in the box that appears. iMovie will open in a new blank library. Reopen it in your old library to get back to your projects. Then see if your lower sound clip plays. You may need to reboot to set the changes.
-- Rich