The way PLAY, BEEP, SOUND, _SNDRAW, or any sound command really, are implemented, is that they push data into a sound buffer that gets output to the sound card. The only way to stop a PLAY/SOUND buffer is by letting it end or by actually closing your program with SYSTEM.
If you use sound files (open with _SNDOPEN, played with _SNDPLAY, etc) - you can use _SNDPAUSE and _SNDSTOP commands.