How to use kAudioSessionProperty_OverrideCategoryMixWithOthers

Since iOS 6.0 you can use

[[AVAudioSession sharedInstance] setCategory:AVAudioSessionCategoryPlayback withOptions:AVAudioSessionCategoryOptionMixWithOthers error:nil];

Swift 5:

try? AVAudioSession.sharedInstance().setCategory(.playback, options: .mixWithOthers)

You can achieve this. The principle is to deactivate your audio session first, setup all the properties of your audio session, and then active audio session. In this way, the music playbacked in other app will not be silenced.

// Initialize audio session
AVAudioSession *audioSession = [AVAudioSession sharedInstance];

// Active your audio session
[audioSession setActive: NO error: nil];

// Set audio session category
[audioSession setCategory:AVAudioSessionCategoryPlayback error:nil];

// Modifying Playback Mixing Behavior, allow playing music in other apps
OSStatus propertySetError = 0;
UInt32 allowMixing = true;

propertySetError = AudioSessionSetProperty (
                       kAudioSessionProperty_OverrideCategoryMixWithOthers,
                       sizeof (allowMixing),
                       &allowMixing);

// Active your audio session
[audioSession setActive: YES error: nil];

This solution works for me. But from your description, You mentioned that you did try activate/deactivate the session before/after changing the category but lead to other problems. Did you do the deactivate/activate in the exact same way as above? If so, provide more information about other troubles and maybe I can help.