It depends where you put your BackgroundMusic item – it will start playing in onCompleted by default when autoPlay is set to true. You could also set autoPlay to false and then trigger the play with the Timer approach you already mentioned.
If you put the sound loading to a Timer where the Scene is drawn (trigger the onCompleted for that), you should be fine on all system. In general, it is a good idea to keep the inital loading time low. You can accomplish that with the Loader item, where the scenes are not loaded at app startup, but only when you set the source property. It is done this way in the updated versions of Squaby and Chicken Outbreak which will be available this week as part of the major update.