Legend, thank you Mark! I’ll have a read and get my teeth into what you’ve done to see how it works.
On initial testing, I’m only hearing Sound 1 which fades out nicely but Sound 2 doesn’t fade in - this is on Firefox. Chrome and Safari don’t seem to like the audio at all but I was experiencing that before anyway.
My goal is to feature this in an iOS app which is a whole other can of worms but I’m guessing if it works well in a browser then it should convert over ok via one of your Xcode templates with ‘Allows Media Auto Play’ activated? I’m clearly over-simplifying this though so feel free to shoot me down 
Thanks again!