Have you updated from 12 to 12.1?
Whatever the case, this is a known issue that hits some, that is, the loss of other audio after the addition of narration using the Premiere Elements Narration Tool.
Please see the the solution of Llamadude99 and see if it works for you to remedy the situation within Premiere Elements rather than resorting to creating the narration in another program and then importing them into Premiere Elements.
We will be watching for your results.
Llamadude99's solution worked. Too weird. Thanks
We are so grateful to Llamadude99 for coming up with the remedy for that Audio Loss/Narration Tool narration issue which I am still not able to reproduce in order to work on it first hand.
Your follow up first hand information is a great contribution to documenting this issue and fix. Many thanks.