@janmichele this "to be worse on certain channels" might actually be more of an issue with where you receive your TV signal from. Reset your cable, sat or any other device you are using as a source, first. Then you want to consider replacing the signal cable i.e. HDMI cable.
Now, if none of that work, check your TV settings and see if you have options to change the audio settings in the TV menu. Since we do not know what exact model your TV is, we can't tell you how or where to find those settings :-)
If at last none of that works and your TV has not getting any better audio, then the issue could be with either the speakers or the audio IC. TV's are not really well know for great audio ;-) Try a pair of headphones, if your TV has a jack for it. See if your TV sounds better with those. If it does, check the speakers. See if you can find a good working speaker and briefly connect it to your TV. If your TV now sounds better, you may just have bad speakers. If none of that improves your audio, then I would suggest a new main board since that is where the audio IC is located. Of course, if you are okay with soldering small components on a circuit board, we could try and find a schematic for you so you could repair the board itself.