What computer operating system is your Premiere Elements 12 running on?
By "my library" I am assuming you mean to have your own sound effects available to you under the Expert and Quick workspaces, either
a. Action Bar Menu at top of workspace/Audio/Sound Effects
b. Audio Tab at bottom of workspace/Sound Effects
This does work for Windows, but I am not sure if it is applicable to Mac.
1. Create your Sound Effect as a .wav file. Name the file....name_prv.wav. In this example bang_prv.wav
2. Go to
Local Disk C
and in the All_Lang Folder create a folder named "Mine". In the "Mine" Folder create a folder named Bang.
In the Bang Folder, place in the Bang Folder the bang_prv.wav plus a copy of the metadata.xml copied from one of the other Adobe
After you close out of there, you should find your sound effect available for use in a project under the Action Bar Menu or Audio Tab route.
I have not experimented with sound effects duration. This quick look used a 4.139 seconds sound effect.
It might be just as easy to create your sound effects as wav files, store them in a folder on hard drive, and, when needed, import one into a project with Add Media/Files and Folders.
Please let us know if this worked for you.