Sound on mobile

I have a background sound which I play in onLoad function of main scene. On computer browser it works, but on mobile it doesn’t. And I have also a SFX for a button click, so then when I click that button the sfx works and I change the scene where all sounds are working.Then I come back to the main scene again where the background sound starts playing. So what is the difference between mobile and desktop browsers? I even tried to add a 5 seconds delay for playing it(supposed some there is some decoding time), but still it doesn’t play until I have an interaction with screen like button click. Is it a bug or am I doing something wrong? Something connected with the focus during the launching of the game?