Sorry, no other ideas – in my experience, when it comes down to these SDK-related volume fluctuations it’s good to go with defaults. It’s an area where there is basically no API contract from Apple (like, if you read the audio mode and audio unit type descriptions they usually describe categories of apps and use the word “may” and refer to “signal processing”, rather than describing concrete behaviors, DSPs, or expectations). I had an unexpected development with really low output volume and VPIO in a very early version of OpenEars which led me to the general conclusion that default audio gets the most QA coverage and has the most “contract” to it, and even when it isn’t ideal for the application, it’s often the prudent call for that reason.
Since you are going to be adding in new API’s here, can I request an API to disable Bluetooth?
Sure, nothing speaks against it now since it should be semantically and behaviorally very similar to the mixing opt-out. I’ll add a ticket and barring any unexpected discoveries related to the feature that would cause me to revise my idea that it is straightforward, I will add it at the same time.