on macOS, the command PLAY always play asynchronously.
If you want to play a sound synchronously, you must call afplay with LAUNCH EXTERNAL PROCESS
var $cmd; $err; $fileName; $in; $out : Text
$fileName:=Select document(""; ".mp3"; ""; 0)
If (OK=1)
$cmd:="/bin/sh"
$in:="afplay "+File(Document; fk platform path).path
LAUNCH EXTERNAL PROCESS($cmd; $in; $out; $err)
End if Take a look at the use of SystemWorkers with afplay to manage a playlist