What computer operating system is your Premiere Elements 12 running on? Have you updated 12 to the 12.1 Update, using an opened project's Help Menu/Update?
To your immediate problem....there is a known issue with Premiere Elements wherein the sound is lost on numbered audio tracks and soundtrack when narration with the program's Narration Tool is added to the project. I suspect that you have fallen into this issue reported by several, but not all.
A remedy has been given by the ID Llamadude 99 and this remedy as well as some modifications of it have worked for most to overcome the problem.
Please read the following for the details, and then let us know the outcome.
We will be watching for your follow up.
Thank you for your answer!
I did the update to 12.1, and now it's working again!
Great news. Great job.
It was interesting and important to learn that the 12.1 Update was the solution and you did not have to use
the Llamadude99's remedy or variations of it to overcome the sound issue. Important follow up.
Best wishes as you move forward in your Premiere Elements projects.