Open a command prompt and enter
then press F6 and select your sound card.
Use left/right arrow key to highlight ‘Channel’ at the bottom, then use up down key to select the number of speakers you have e.g. 5.1 so you should select 6. Press escape when done.
To restore this after a reboot type
sudo alsactl store
Just make sure you have ‘5.1 out’ selected in the desktop volume control – settings for your audio card and you should now have full 5.1 sound.
Let me know if this works for you using other modes like 7.1 or just 4 speakers etc. It should.