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INT 1Ah,  80h (128)      Set Up Sound Multiplexer                        PCjr

    Determines the source to be used for sound generation.

       On entry:      AH         80h
                      AL         Sound source
                                    00h - 8253 channel 2
                                    01h - Cassette input
                                    02h - "Audio in" line on I/O channel
                                    03h - Complex sound generator chip (TI
                                 76496)

       Returns:       CF         Set if error; else cleared

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       Notes:         This service is available only for the PCjr. ATs,
                      XT-286s, and PC Convertibles will set the Carry
                      Flag, thus reporting an error state. Previous
                      machines will report unpredictable results
                      (including the state of the Carry Flag).

                      On power-up, the default sound source is the 8253
                      programmable-timer chip, via channel 2.