INT 15 - TopView - "SOUND" - MAKE TONE
        AX = 1019h
        BX = frequency in Hertz (0000h = silence)
        CX = duration in clock ticks (18.2 ticks/sec)
Return: immediately, tone continues to completion
Notes:  if another tone is already playing, the new tone does not start until
          completion of the previous one.  Up to 32 tones may be queued before
          the process is blocked until a note completes.
        in DV 2.00, the lowest tone allowed is 20 Hz
        if CX = 0, the current note is cancelled; if BX = 0 as well, all queued
          notes are also cancelled