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The mute LEDs on the I/O cards are not connected to the mute switches on the channels that are patched to those particular outputs.  The LED mute circuitry is built in to the IO cards very close to the actual outputs, and only switches the mute LED off when it detects a change of signal level.  It is only an LED indication, there is no mute circuitry built into the card that unmutes when a signal is detected.