For 5.1 channel tracks, allow the panner to be turned off, mapping the channels as-is to a 5.1 channel output.
Audition doesn't allow multitrack editing of existing 5.1-channel tracks without the panner steering the sound around, by default to only the front left and right channels. There's no option to simply disable the panner. In comparison, Premiere allows mixing 5.1 channels with no panning; it's the default behaviour.
This isn't actually the panner's fault, it's that 5.1 multitrack sessions default to being filled with stereo tracks for some godforsaken reason. If you create a new 5.1 track in the session then add the 5.1 channel there this doesn't happen. Expanding the panner to cover all channels just expands the stereo downmix of the 5.1 track you dragged in to cover all of the channels, it doesn't actually do what you'd think.