Full Downmix of All Audio Tracks in Player

We regularly review content with 8 audio tracks (surround 1-6, a matching stereo full mix on 7+8). 

Selecting the active audio tracks in the player seems to only allow playback on 2 specific tracks from the file, or the complete surround mix on tracks 1-6.

To catch disparities between the surround mix and stereo mix on 7+8, it's useful for us to be able to choose playback of all 8 tracks simultaneously. Whether it's 1 or 2 frame sync issues between the surround and stereo mix we receive, or (in rare cases), there is sometimes a serious difference between the VO on track 3 of the surround mix, compared to the VO on the stereo full mix on 7+8. 

Issues like that are much easier to catch if a full downmix is played concurrently.