First of all you will need the sound you want to add in .wav format.
Secondly you will need UnrealEngine for this tutorial
1. Open the tab like the picture below:
2. Use the import option to insert your custom sound
3. DONT FORGET TO BE ON .wav FORMAT
4. Then this window will popup
5. Rename the "Package" into the name of the file you want to make. Example:
6. Then press ok and you will have something like this:
7. Finally open the "File" tab and press save like the picture:
~ Your custom .uax file is ready! ~
L2FileEdit: (Based on the tutorial names) TutorialSounds.Levelupcaster
Credits: SoFaKi