The problem is Bluetooth audio, from all sources, is far higher than 2ms latency. The best codecs seem to be (per sources like this -- http://stereos.about.com/

The problem is Bluetooth audio, from all sources, is far higher than 2ms latency. The best codecs seem to be (per sources like this -- http://stereos.about.com/od/glossary/fl/What-is-aptX.htm http://stereos.about.com/od/glossary/fl/What-is-aptX.htm) 40ms or so. "Typical" Bluetooth audio seems to have a latency of 150ms-200ms. Musicians can compensate to some degree of latency, but not when your audio is delayed by approx. an 1/8th note at 140bpm. (1/8th note at 140bpm = 214ms). Even 40ms isn't terribly tight. So rule Bluetooth audio out for this. The dongle is an option, of course, but it probably is not as robust as a soldered-in 3.5mm jack. I'm sure in-ear monitoring with an iPhone is unfortunately too much of a niche application for Apple to care much, though...