Using iMovie on iPhone with VoiceOver

Hi Applevis users Kaushik from India here, I am using iMovie to edit my videos.
I am facing one issue, whenever I want to add the background music for my clip after I join couple of videos my video is not allowing me to add background music even though if it adds it is going to the background I cannot able to paste my audio background track wherever I want to do help with iMovie editing