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Int 1Ah, 80h            Set Up Sound Multiplexer                         PCjr

    Determines the source to be used for sound generation.

        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)

        Return  CF   Set if error; else cleared

    ------------------------------------------------------------------

    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.