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What you describe is simply the inferiority of built-in TV speakers. This is why many of us send our TV audio through our main stereo system. TV sound can vary enormously. The audio in the baseband channels is typically highly compressed and forward. Premium movie channels are much better with less compression and more resolution. HD channels have no signal processing and the audio is excellent, on par with DVD soundtracks. The music channels are MP3 quality at best. My Motorola cable box has an on-screen audio compression option which I have set to "None".
It is technically impossible for the FM audio from a standard television broadcast to be as good as what good CD playback has to offer in terms of both frequency extension and dynamic range. I agree with Blueswan that it just sounds so much better through your headphones than the regular speakers that you are temoprarily fooled that it is as good as your CD, unless of course there is a problem with the quality of the CD playback.