INT 2F - Kingswood TSR Windows - SOUND BEEPER
        AX = 3901h
        BL = 0Ah
        DX = sound divisor, or 0 for silence.
             (divide 1843200 by required frequency to get value for DX)
        CL = sound length in 18.2 Hz clock ticks
Return: AX = error code (0000h if successful)