One way to get simple and configurable audio is to have a matrix-mixer.
Example: mix a 2-channel old movie to the correct center mono. Adhere to Academy rules, which is 3-channel mix.

 output channels
input channels left right center
left

0.5
right

0.5
center


surr left


surr right


back left


back lright


LFE







silence
 silence
 mix


Example: an ordinary 2-channel stereo to 5.1 without any mixing.

 output channels
input channels left right center surr left surr right LFE
left 1




right
1



center





surr left





surr right





back left





back lright





LFE













mix mix silence silence silence silence

It might be good to grey out or not draw the input channels that are not available.
With this it can also be possible to do matrix-decoding from 2-channel sources.

/Adam Klotblixt