This could be an auditory type fear. An auditory type of fear is when your dog is startled and becomes fearful of a sound. Here are some things you can try to help your dog;
Create a journal and write down each time your dog reacts to the TV to see if you can isolate the sound so you know what the trigger is.
Start counterconditioning and desensitization like in this FREE video on How to Desensitize Your Dog to Loud Sounds here: https://vimeo.com/410212531
You can also check out Truly Force Free Animal Training Understanding & Helping Your Reactive Dog Webinar here: https://trulyforcefree.com/product/webinar-understanding-helping-your-reactive-dog/