Overlapping time audio cues won't work

[robertbryer]

If two audio cues are simultaneously active, it will cause problems with the default audio output - On a leaf node becoming inactive, the default audio output will be switched on, which will work when one leaf node is active but if two are, it will play the default output when the second is still active. Instead we should maintain a list of active leaf nodes, so we only turn on the default when 0 leaves are active. This might be lower priority to fix because although overlapping audio cues are allowed, they're a bit of an edge case.

[nigelmegitt]

I think we should prohibit overlapping audio cues by specification. To be confirmed as part of the community/working group activity.