I have an iPad and an iPhone and experienced the same problem no matter what browser I use
, not sure if this is a Construct 2 problem or a iOS problem ( 
Ashley )
but
when music is placed in the sound file it seems to accept volume controls.
The problem is that the entire file needs to be downloaded before it plays when it is placed in the sound file and on mobile devices it takes a while to download larger files. also, very large files will crap out the browser
so what is really needed is a option to preload all sounds before game starts as an option within C2.
So you have to think of a work around for that.
For example,
I have made my own 'loading' layout, and I placed a loading bar with a dog running across the screen to a doghouse which takes about 20 seconds , when the dog gets to the doghouse it goes to the next layout
( here is a cool place to find these types of things for loading bars etc http://preloaders.net/en/
in that event sheet I play all my sounds in the entire game at volume -10000 ( no sounds are heard, even on iOS devices , sounds within the sound file anyway) and just to slow down the process a tiny bit I place a wait event (0.12) in between every two sounds.
a bit cumbersome, but it works.