When an email notification is received, XiiaLive volumn is changed - much lower. This is happening on Android Tablet connected via WiFi. I've disabled volumn

That for the feedback. Generally when email notification sounds are triggered by the OS XiiaLive would duck the volume a little and after the sound is played it would raise the volume back up. Did you notice the volume regain at all? 

What device do you have and what android version is it running?

I also have the problem, but only a few times. It happened much more often in earlier versions of XiiaLive.

When I get a Facebook Messenger notification sound, the music from XiiaLive fades out and sometimes it doesn't fade back in again.

I also don't know why Facebook Messenger interrupts XiiaLive at all - I enabled "Ignore audio focus" in XiiaLive?!

Thanks for the info. Yes, Ignore audio focus should do the trick.

Yeah, it should, but it doesn't :)

So is this a bug? Or is Facebook even lowering the system media volume manually which would not use Audio Focus at all?

Please make sure you have XiiaLive v3.0.3 installed. On that version we changed how we manage audio focus. If ignore audio focus is enabled all audio events are simply ignored. If you are hearing the audio lowering then it must be the Android OS automatically for notification events.