For people still facing this issue in 2019, I have found out that Roku firmware (version 9.1.0 in my case) returns an empty string unless the audioTrack property on the Video node is set explicitly by whichever means available to final users or developers.
On a Scene Graph Video node, this means that a video that just starts playback will not produce a usable value for the audioTrack property, but that the Video node will start reporting the correct value after setting it either via the * key on the remote control, or by setting the audioTrack property itself, which is something I have done (as a channel developer) using a custom command based on ECP roInput.
Obviously, this is unexpected behavior and as such I would consider it to be a bug on the firmware.