Complex sentence: By her own choice, Galli-Curci's singing was largely self-trained, obtained by listening to other sopranos, reading old singing method books, and practicing piano exercises with her voice instead of a keyboard.
Split into simpler sentences: By her own choice, Galli-Curci's voice was largely self-trained. She honed her technique by listening to other sopranos, reading old singing-method books, and doing piano exercises with her voice instead of a keyboard.