I am having the same difficulty, which essentially hampers my ability to use the OS4 for mobile use, one of the primary reasons for my purchase. Unless I missed something, the SharkRF Link app does not appear to allow selection of any audio output device (i.e. Bluetooth.) It defaults to speaker mode on my iPhone.
Other IOS apps, such as those used when playing music, or when taking a telephone call, allow for selection of an audio device, including those which function via Bluetooth such as headsets and wireless speakers (e.g. including those integrated into a vehicle's multimedia sound system.)
Thoughts on what we are doing wrong in terms of iPhone configuration, or perhaps plans to add Bluetooth functionality to the iOS version of the SharkRF Link app?