https://developer.apple.com/library/archive/qa/qa1754/_index.html#//apple_ref/doc/uid/DTS40011281
Q: Can you explain the difference between calling the AVAudioSession method overrideOutputAudioPort: with the value AVAudioSessionPortOverrideSpeaker and using the category option AVAudioSessionCategoryOptionDefaultToSpeaker with setCategory:withOptions:error:.
A: The difference is that setting the AVAudioSessionPortOverride by calling overrideOutputAudioPort: is more transient than using the category option AVAudioSessionCategoryOptionDefaultToSpeaker.
Important: The use of both AVAudioSessionPortOverrideSpeaker and AVAudioSessionCategoryOptionDefaultToSpeaker are only applicable to the AVAudioSessionCategoryPlayAndRecord category.
Calling overrideOutputAudioPort: and setting the AVAudioSessionPortOverride to AVAudioSessionPortOverrideSpeaker is a way of temporarily overriding the output to play to the speaker. Any route change or interruption will cause the audio to be routed back to its normal route, following the last-in wins rule. Think of using overrideOutputAudioPort: in terms of what you might use to implement a Speakerphone button where you want to be able to toggle between the speaker (AVAudioSessionPortOverrideSpeaker) and the normal output route (AVAudioSessionPortOverrideNone).
Note: This property is intended to allow 3rd party applications to mimic the behavior of a Speakerphone button and therefore may change the input route as well as output route. For example, setting the AVAudioSessionPortOverride to AVAudioSessionPortOverrideSpeaker while a headset is plugged in will cause the route to change to built-in mic / built-in speaker.
The preferred way to implement Speakerphone functionality is to use MPVolumeView's Route button.
In contrast, AVAudioSessionCategoryOptionDefaultToSpeaker modifies the routing behavior of the AVAudioSessionCategoryPlayAndRecord category so that audio will always route to the speaker rather than receiver if no other accessory such as headphones are in use.
When using AVAudioSessionCategoryOptionDefaultToSpeaker, user gestures will be honored. For example, plugging in a headset will cause the route to change to headset mic/headphones and unplugging the headset will cause the route to change to built-in mic/speaker (as opposed to built-in mic/receiver) when this override has been set.
In the case of using a USB input-only accessory, audio input would come from the accessory and output would route to headphones (if attached) or speaker if headphones are not plugged in. The use case is simply to route audio to speaker instead of receiver in cases where the audio would normally go to the receiver. This option is a modifier for the category.
Route changes and interruptions will not reset this override. Only changing the AVAudioSession category will cause this option to be reset.