kReverb2Param_ используется для AudioUnitSubType Reverb2, а не для MatrixReverb.
Вы можете попробовать установить следующие параметры:
// Parameters for the AUMatrixReverb unit
CF_ENUM(AudioUnitParameterID) {
// Global, EqPow CrossFade, 0->100, 100
kReverbParam_DryWetMix = 0,
// Global, EqPow CrossFade, 0->100, 50
kReverbParam_SmallLargeMix = 1,
// Global, Secs, 0.005->0.020, 0.06
kReverbParam_SmallSize = 2,
// Global, Secs, 0.4->10.0, 3.07
kReverbParam_LargeSize = 3,
// Global, Secs, 0.001->0.03, 0.025
kReverbParam_PreDelay = 4,
// Global, Secs, 0.001->0.1, 0.035
kReverbParam_LargeDelay = 5,
// Global, Genr, 0->1, 0.28
kReverbParam_SmallDensity = 6,
// Global, Genr, 0->1, 0.82
kReverbParam_LargeDensity = 7,
// Global, Genr, 0->1, 0.3
kReverbParam_LargeDelayRange = 8,
// Global, Genr, 0.1->1, 0.96
kReverbParam_SmallBrightness = 9,
// Global, Genr, 0.1->1, 0.49
kReverbParam_LargeBrightness = 10,
// Global, Genr, 0->1 0.5
kReverbParam_SmallDelayRange = 11,
// Global, Hz, 0.001->2.0, 1.0
kReverbParam_ModulationRate = 12,
// Global, Genr, 0.0 -> 1.0, 0.2
kReverbParam_ModulationDepth = 13,
};