Dell Optiplex 2020 smal form factor sound only from internal speaker

Hi  Sound only from internal speaker, when I plug external speakers to the green port on the rear all sound is muted. How do i disable internal speaker but without muting the external speakers. This must be so simple am I missing something obvious. Ps pc running Win 10 pro 64 bit with 16 gig of ram.  Powered speakers are ok tested on a known working machine and plugged into the green port on the rear of the micro tower. Speakers are not muted