You can add background music to new responsive slideshow skins like:
In order to use this feature please follow the steps below:
Create your Cincopa gallery.
Choose one of the slideshow skins.
On step 2 Custom Skin page click on “Advanced” tab and set the “Background music volume”.
On Upload files step add the files you need to be displayed in your gallery additional one audio file that will play in background.
NOTE: In order to work you need to add the music file (just one) as the first file in the gallery.
Drag & drop files to rearrange your gallery items - can be done on both List and Grid views: