Are you getting the sound directly from the TV, or are you using a third party device such as an amplifier, soundbar or speaker? If so is it connected via HDMI or optical cable?
If using a third party device have you added it to the 360 via Settings > Audio & Video > Pair remote with devices > Audio Set up? which has three options. Try them all, but usually PCM2.0 works with older TV's
Also if using a third party device via HDMI and it's all set up, check the Settings > Audio & Video > Volume keys control. Pressing the OK button switching sound control between the TV and the 3rd party device (clicking the back button on the remote exits that screen and leaves the sound control set to whichever type of device is showing on that screen.
If none of the above applies then please state the make and model number of your TV (as previously requested) and any 3rd party sound equipment, and how it's connected.