Hi lzhang,
After looking at the source (specifically at the anim montage factory in the editor source), it looks like it is by design that an anim sequence doesn’t know anything about the montage it is in. That would make sense to me because a sequence can appear in multiple montages or be played on its own.
To avoid having to edit the engine’s source, the simplest solution would be to duplicate the sequence. You could have one version that has the notifies to play the sound, and another without any notifies. If that wouldn’t work for some reason, you could also have some kind of global state about whether or not notifies should play sound and update that as needed.