0dB is pretty loud, no? IIRC it's the loudest sound expressible on an audio CD. (Decibels are a log scale - true silence would be minus infinity).
With regards to a CD, the decibel measures the gain rather than the volume. 0dB in this context simply means "full volume", while -10dB means 10% of full volume, and -20dB means 1% of full volume.