I use exactly the same method for input volume adjustment, albeit for just one 
mixer unit;
AudioUnitSetParameter(myPlayStruct.mixerAU, kMultiChannelMixerParam_Volume, 
kAudioUnitScope_Input, 1, 0.0f, 0 ) ;

In this case the volume is set to zero (0.0f), and it works fine for my 
user-controlled muting.
Have you triple checked your AUGraph topology, that might be allowing things to 
seemingly work, but not be actually hooked up the way you intended.
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