Edit Screen > U-mute Slide Properties > Background Music
You can set background music that is used throughout the entire project and background music that is used only in specific slides.
Edit Screen > U-mute Slide Properties > Background Music > Select
After pressing the "Select" button, you can preview the music and apply it; if the same audio is already used in consecutive slides, it will continue seamlessly.
Edit screen > Slide Properties on the right > Background music > Volume
You can decide on the appropriate volume while previewing.
If the original audio is separate and you are only adjusting the volume, the global background music will automatically apply fade in/out effects, so the volume is adjusted smoothly.