After listening to your high-volume car stereo for 15 minutes, you fail to realize how loudly the music is blasting. This best illustrates:

The phenomenon described in the question illustrates sensory adaptation, which is the process by which our sensory receptors become less sensitive to constant stimuli over time. After being exposed to the loud music for 15 minutes, your auditory system has adjusted to the high volume, leading you to become less aware of the noise level. This adaptation occurs because your body is designed to filter out unchanging stimuli, allowing you to focus on changes in your environment that are more important for survival or attention.

In the context of the question, although the music is still playing at a high volume, you are no longer aware of it because your perception has adjusted to the ongoing stimulus. This decreases the perceived intensity of the sound as you get used to it, demonstrating sensory adaptation at work.