Headphones not playing sound on Beamng but on monitor???

Other games play it through the headphones as normal, however this, this is playing through the monitor

 

Have you made sure that your headphones are the default playback device, this sometimes happens with me.

To access this menu right click on the sound icon and select playback devices.

 

Another tip, you can switch between the 2 if you'd like.

In the game go to Options>Audio then choose your Audio Device