Fix issue with background music not playing on iOS without first setting the volume.
Fix issue with background music not playing on iOS without first setting the volume.

The volume was not being set correctly if there are not preloaded background music.  This was changed to set a volume parameter no matter if there were preloaded background music or not.

See issue https://github.com/mono/CocosSharp/issues/272

haithemaraissia/CocosSharp,MSylvia/CocosSharp,MSylvia/CocosSharp,mono/CocosSharp,mono/CocosSharp,zmaruo/CocosSharp,TukekeSoft/CocosSharp,zmaruo/CocosSharp,haithemaraissia/CocosSharp,TukekeSoft/CocosSharp