Thanks for reaching out.
As of now, only one piece of music (file) can be added to a video project. It's something we are looking into. If you want to share for others to upvote, you can submit your suggestion at bit.ly/expressuser
As a workaround, you can compile multiple songs you want to add to the video project in a single file using other Adobe apps and then use that single file in the Adobe Express video project.
Let us know if it helps.