You probably know that older movies or shows on the TV are sometimes easier to understand because the audio wasn't as overly processed. Nowadays, producers often emphasise certain aspects of the audio signal for television by artificially 'companding' (compressing and expanding) the audio signals.
By doing this, the maximum amplitude is limited to meet broadcasting standards, but the lower intensity sounds can be made louder.
This 'compansion' greatly reduces the dynamic range of the audio signal and removes many of the vital cues needed for the hearing impaired to understand what is being said on television.
Many TV shows are now produced in a format for use with surround sound channels (5.1 format) as standard. When these channels are re-mixed and transmitted with the TV signal, they too are companded and the dynamic range is reduced.
Hearing aids also reduce the dynamic range of sound and, when used to listen to TV, can make it even more difficult to understand.