Very easy:
Load the Firmware you want to Change
Then use Export > All sections to split the Firmware into ist partitions
Afterwards use RFS > Export to export the files from RFS Partition
Now you can modify the Logos/sounds which are located in the "rfs" Folder,
Please Keep the dimensions of the Pictures and save them with paint since it removes all EXIF data. If you want to remove sounds open the soundfiles in anEditor, delete the Content of the whole file and add a single space and save it.
When you are done you can recompile the Firmware using These instructions:
RFS > Build/recompile
RFS > Include
Build > Firmware binary
After this you get a new file called aXfirmware.bin
Rename it to fit your devices requirements and Flash it.
Check the Firmware filesize before flashing. It should differ only a bit (max. ~200 KBs)