The 12v battery isn't used to start the vehicle and therefore can be much smaller than for a traditional vehicle, which saves space and weight. The downside is that it's electrical capacity is much lower and therefore goes flat quite quickly. Draining it below 70% causes minor damage to the battery, lowering the capacity, which means you are more likely to flatten it again next time and so you get into a vicious circle.
When using any power, e.g. listening to radio etc, you should always have the vehicle in 'Ready' so the hybrid system is providing power to the vehicle, and actually charging the 12v battery. In 'Ready' if the hybrid battery goes low the petrol engine will run for a couple of minutes to charge and then shut down again.