Thank you for playing! :)
Regarding sound and music, there is an option to toggle sound and music (separately), so in a way there is a "volume control", but only with 0% and 100% as options. Lack of indicators for enemies too far to be on screen is indeed emulate difficulty of such games (but also all enemies spawn and move in a simple deterministic manner when entering a new area (they are reset to the default state (except for bosses, which stay defeated)), so the idea is that a skillful player might in principle use this to predict their locations).