NTLite forum doesn't provide help on how to modify Windows system resource files, but you can find a few online guides explaining how to update the Windows startup sound using ResourceHacker.
To replace imageres.dll.mun in the image:
1. Load the image.
2. From the Post-Setup screen, Add / File (Before logon) and select your modded imageres.dll.mun
3. Change the Destination path to "\Windows\SystemResources".
4. Apply your changes.
