How to make emerson lf20em4 play sound trough headphones only?

Have you looked in the manual for advice ?

try your headphones on another device to ensure the headphones are not the problem. ensure any mute or volume switch on the headphones are turned up. and ensure the batteries in the headphone are charged. check attn try both 0 and -8db setting,
check both headphone outlet and tv speaker outlet of smart tv. in TV settings ensure the audio out setting is set to ON so that the jacks for audio to external speaker are switched on.

i hope this was useful.

oh wow ummm try messing with in game sound setting

There could be a setting on this model in the menu to set the type of scene you are watching...like movies, sports, etc. Sometimes these settings will affect the sound. Also check all that is included in your sound menu. Test each setting in the sound menu.

Double check that the adapter you bought is designed for stereo. The easiest way to check this is count the number of metal bands you can see on the adapter, stereo headphones will have 3 metal bands separated by 2 plastic bands, whereas mono will have 2 metal bands separated by a plastic band.
If you accidentally bought a mono adapter, this will cause sound to only play in one ear.

It sounds to be you have a banned xbox. No matter what you do to it you won't be able to play online. The only thing you can do to fix the problem is to buy a new one and don't trust buying used unless you see have demoed it online first.

you have to enable it to play audio through your headset and tv

Try connecting earphones or headphones to the 3.5mm Headphone socket on the TV and check this works correctly, i.e hear TV when watching TV, hear DVD when watching a DVD etc.
If all is OK, purchase a 3.5mm Jack to L/R Audio/Phono cable. Connect the 3.5mm jack in to the headphone socket on the TV and connect the red/white phone ends to the 5.1 surround system.
It should solve it. SCART out is not always live in all sources so is not a reliable way to connect.