To create a video with multiple music, start with the same procedure for creating a video. Once you've added a set of photos for a particular music, proceed by selecting the button ‘Add Another Music'. Select a desired audio and add the content to be played under that music.
PS: If you do not add content (photos/text) to this extra audio, it will not be played in your video.
After adding content to each block, your project should look something like this.
Also, please keep in mind that the music files will not be played together. One music will be played till the content for the music block exists.