Here is a link to instructions to determine if the remote is transmitting on the non working functions. http/www.wikihow.com/Check-if-a-Remote-Control-is-Transmitting-an-Infrared-Signal
You can also try a remote that can be programmed to operate your TV like a cable box remote to see if that will change the volume of the TV,
If the remote isn't working then you could try cleaning the contacts of the remote but this is often not possible. You should be able to get a replacement remote.
Has anything been spilled on it? The buttons on a lot of remotes are the switch closing bridge. If something is spilled it insulates the circuit board from the key and it won't function.
Here is a link to instructions to determine if the remote is transmitting on the non working functions. http/www.wikihow.com/Check-if-a-Remote-Control-is-Transmitting-an-Infrared-Signal
You can also try a remote that can be programmed to operate your TV like a cable box remote to see if that will change the volume of the TV,
If the remote isn't working then you could try cleaning the contacts of the remote but this is often not possible. You should be able to get a replacement remote.