cant add music to imovie project after i finalized and exported, but want to edit some things

I made an imovie project and exported it as mp4 to show my family outside imovie to play in quick time so i didnt have to have my extermal harddrive attached that had alot of the movie clips. they made suggestions on what to change a little bit and now i want to make those changes and move around some stuff. There are a few sections where i want to add in some music in the background and i did it totally fine before, but now that i am going back in and changing things i can add the music clip in the project, but i cant hear the music during playback, it shows up in the project area like it should, but no audio during playback. is there some feature that gets switched after i share/export it that i need to uncheck or something to edit it. I also tried using the clip in another project that i am working on and have not shared yet and it works perfectly fine in that. so im assuming its not the source clip thats the problem. hopefully somebody can help! thanks!