 <p>I used my apple developer support for this. Here's what the support said : </p> <blockquote> <p>The presence of the Voice I/O will result in the input/output being processed very differently. We don't expect these units to have the same gain levels at all, but the levels shouldn't be drastically off as it seems you indicate.</p> <p>That said, Core Audio engineering indicated that your results may be related to when the voice block is created it is is also affecting the RIO instance. Upon further discussion, Core Audio engineering it was felt that since you say the level difference is very drastic it therefore it would be good if you could file a bug with some recordings to highlight the level difference that you are hearing between voice I/O and remote I/O along with your test code so we can attempt to reproduce in house and see if this is indeed a bug. It would be a good idea to include the results of the singe IO unit tests outlined above as well as further comparative results.</p> <p>There is no API that controls this gain level, everything is internally setup by the OS depending on Audio Session Category (for example VPIO is expected to be used with PlayAndRecord always) and which IO unit has been setup. Generally it is not expected that both will be instantiated simultaneously.</p> </blockquote> <p>Conclusion? I think it's a bug. :/</p>