How to mix sounds from different audio units in iOS?

Question

I have an application which has VoIP and at the same time it renders customs sounds.

The VoIP audio unit is set as follows:

au_description.componentType          = kAudioUnitType_Output;
au_description.componentSubType       = kAudioUnitSubType_VoiceProcessingIO;

While the one which renders the sounds is:

au_description.componentType          = kAudioUnitType_Output;
au_description.componentSubType       = kAudioUnitSubType_RemoteIO;

I have an issue while mixing both sounds (voice and custom sounds). The rendered sounds are played at a lower volume when the voice has started (even if it is currently muted). Comparing the rendered sounds played alone, and the same sounds during a VoIP call, the result is 13 dB lower.

Thanks, Leo.