I'm not sure what the difference between a digital track and channel would be.
All I know is that later games (SCI11 and higher) use Audio resources to do non-speech digital sound effects, that may match IDs with Sound resources. That may be why you couldn't find anything in SQ6's sounds, all its digital effects would be in another resource type entirely.
In earlier games, these digital sounds are part of of the audio resource itself, like in the provided EcoQuest (disk ver.) example (*chomp* OUCH!). That's all I know.