All of our senses acclimate to continual sensations, and our brains filter them out as background stimuli.

When a noise is continual, it will fade into the background -- people don't "notice" air conditioners running, traffic noise, wind blowing, and all the other ambient sounds that fill a room. If you stop to think about them, you can certainly hear them, but when they are constant, we easily stop paying attention to them (or at least pay less attention to them).

Similarly, we don't consciously notice our own clothing -- think of how distracted we would be if we were always aware of our clothes rubbing against our skin! We feel the fabric when we first put our clothes on, but very quickly, we stop noticing them. We also don't notice our own breath, no matter how bad it is. Our mouth is literally right under our nose, and yet we don't smell our own breath because we're smelling it all the time.

Our brains have evolved to "filter out" continuing stimuli to ensure that we notice new and novel stimuli. Our brains essentially say "that sound has been the same for quite awhile now -- it must not be anything important, so I'll stop paying attention to it." For primitive humans, this allowed them to focus on novel sounds which could be a predator or some other potential danger, even if the air is filled with the sounds of wind, trees rustling, water flowing, or whatever else might be going on.

So when TV or music is playing, our brains will eventually get used to the noise and stop paying attention to it, however if there is a new and novel sound, our brains will become alert, essentially saying "Something new! There might be danger nearby!" Similarly, if someone were to very lightly touch your arm -- much lighter than your own shirt is pressed against your skin -- you would notice this as well, despite the fact that you don't notice your own clothing constantly touching you.