Sorry to hear about your problem, I will be happy to help. Please be aware that the procedures that are required to solve your problem can be complex and in some cases you will have to replace parts that may be expensive. Solving your problem depends on your ability to provide detailed responses and following directions very carefully. The more information you can provide the better chance of solving your problem. I hope you find the information I provide valuable considering the average price of an in-home Tv diagnosis.
What are you using for the source(s), how are they connected, and is there no sound on ALL of the Inputs?
Try a system AV RESET: With the Tv on, press MENU on the Tv remote, then press 123 ENTER. All the settings in the Tv will be returned to factory and the Tv will re-discover the source(s). If you use the built in Tuner you will have to run the channel scan. See page 82 of your user manual: http://www.mitsubishi-tv.com/pdf/151_153_OG.pdf
If you need further assistance please reply here. Thanks for using Just Answers. Don't forget to click the ACCEPT button if you find this information helpful.
"Sources" means cable box, satellite box, bluray player, dvd, vcr, etc. Have you reset those devices (unplug from power, wait, plug back in)?
What source device(s) do you have connected to the Tv and with what kind of wires? Have you checked for loose connections on the back of the Tv and the back of the source device(s)?
Are you using a surround sound system or just the Tv speakers?
Did you read page 82 of the manual that I linked to? That explains how to do the AV Reset.
If your Tv really has a failure, then a circuit board in the Tv will need to be replaced. A repairman will charge $250-$300 labor plus $470 for the Main board: http://www.encompassparts.com/products/?934C335001
Even if you cannot get the sound on your Tv working with my guidance, I would hope that you find all this information helpful and click Accept, considering a repairman would charge $100-$125 just to come to your house.
The more information you can provide about your system, the more I can help you. Right now all I know is you have a Mitsubishi LT-46151.