I just tried this (on my iPhone 6 running iOS 12.0.1) and am now able to play audio and get speech recognition at the same time. Great job!
There’s only one small issue. The audio volume level for playback is extremely low.
I found this comment online that may provide insight:
When recording audio, or configuring a Record or PlayAndRecord Audio Session, the iPhone switches from using the speaker (on the bottom) to using the smaller earphone (nearer the front top), which produces a lower sound volume. Your app can override this default by setting the Audio Session kAudioSessionProperty_OverrideAudioRoute property to kAudioSessionOverrideAudioRoute_Speaker . See Apple's Audio Session API documentation for details.
Thanks for your help on this.