There's no mistaking the sound of the conventional 12v starter. So it's using the IMA and taking a small amount of energy from the pack to do it each time.
Obviously the 12v starter requires a healthy 12v battery to work. Also, these cars are known to get corrosion on/in the grounding cables in the engine compartment which may also prevent the 12v starter from working even if you have a good 12v battery.
Lift the rear carpet in the back (the section of carpet ahead of the storage bin). You'll see:
Remove that little access cover and you'll find:
