Apple have made background music rather simple in iMovie.
You are allowed only one track, and it just starts at the top.
I have found the only way to do this is to create multiple projects that later are assembled together as a whole.
Not the nicest solution, but it works.
Duplicate the project,
then delete the 30 seconds you want music free, and add the music track,
save this new video out to the camera roll.
Open the origonal project, delete the bit you just added music to, and insert the other video you just made.
This way you can have multiple music tracks too .
I tend to edit the whole video, and then break it into parts to add music and adjust the edits.
Then assemble all the parts again for a final output.
This is reasonably quick for small home movies.
I realise that everytime you save to camera roll you lose quality of video;
but it is minimal and I also tend to archive out the iMovie Projects just in case I want to edit again later.
I tend to use 'Splice' or 'Voddio' if I want to edit more complex stuff and retain quality.