This mod uses heart icons to represent the girl's "health". It starts at full hearts, and then heart pieces will vanish as she gets closer to asphyxiation. If you leave time for the girl to recover then hearts will automatically reappear.
Double-check the "Breathing" option in the "Sounds" section of the in-game "Options" tab. If the "Breathing" option is disabled then the mod will not work. It's okay if the volume slider is set to zero, but the "Breathing" option must be enabled because the game doesn't even calculate the girl's breath level when breath sounds are off.
Please download the linkhearts.txt file and place it in your SDT\Settings folder. The file allows you to customize the behavior of the mod (e.g. number of hearts) and to enable/disable the mod's sound effects.