Plays a sound file.
Under X Windows, the play command must be defined through the user's X resources file with the resource name ``mred.playcmd''; this command string is formatted with the input filename (so the command string should contain a ``~s'' where the filename should be substituted) and executed as a shell command.
Under Windows, only ".wav" files are supported.
Under MacOS, only standard Macintosh sound files (SND) are supported.