When a path including an environment variable is used for the location of a sound file, clicking the "Preview" button will result an a "Invalid Sound File" message stating "Sound file does not exist".
E.G.
Play sound, '%TEMP%\sound.wav'
However, when the command is actually executed, the file will be played correctly(tested using "Legacy Audio" and "Integrated Components").
This is also relevant to the "Play a Random Sound" action's preview feature.
EDIT: Working in v1.6.7.8