iMovie failed audio export to YouTube

I have created a 8 minute clip on iMovie from various video clips. I'm using 2 songs as backing tracks which cover the whole 8 minutes. For some of my video clips I've muted the sound but for others I've left it on (It is definitely still on - I have checked many times and is still on when I preview the clip). Problem is, when I export the clip to YouTube (have tried both 720p and 1080p) I lose some of the audio (not the music - the actual video clip audio) from some of the clips. The strange thing is the music still fades as it would when audio from a video is playing over the top of it but no video audio can be heard. I've even tried to detach the audio from the video and place it separately underneath but no luck with this method either. Is this a bug with iMovie or is there a fix? It's very frustrating. Thank you.