Hi,
Currently the autoplay feature of the playlist system will play a new sound when no other sounds are playing.
Your request make sense and I'll change this behavior in the future so that it only takes into account sounds launched from the playlist.
In the meantime, you could use the instruction system to play sounds one after another. You can drag and drop sound button in the instruction pane, you'll need to set each instruction to autostart to not have to press the GO button.