How to get audio to the side (surround) speakers of a 7.1 system?

    I have a 7.1 audio sound system attached to one of my LibreElec clients. However, since DVDs and TV stations only support up to 5.1, I only get output in the 5.1 subset of speakers. What I need is a way to be able to drive the side speakers with a blend of the rear and front signals. Can someone tell me how to do that?

    Also, are there any plans for incorporating the ability to change the volume send to the rear or side speakers relative to the front speakers? My sound hardware doesn't allow me to change it.

    Thanks!
    Dan