thanks alot thats help my problem

Ochna, you can add a BGM by using the "BGM File" parameter as said in the description above. This option should be the second to last option, with the option "Select File" and choosing a local audio file on your personal device. In case it is still unclear to you, I shall try and re-describe it below.

To choose a file that isn't .webm, .ogg or .ogv (such as .mp3), you must navigate to the folder containing the audio file you wish to use, then type in the full name and extension of the file into the "File name" parameter at the bottom of the window (e.g. "Aira.mp3" without the quotations - unless your file name has it) and click "Open". Then simply restart your Wallpaper, and the BGM should start. You can then adjust the volume using the volume slider option immediately below the BGM File parameter.

how to add bgm?