Here is a explanation of the phrases/words in context:
Übestimme:
Is the highest voice in a musical movement.
in die Stille gelauscht werden:
I am not sure what it means...
I guess it means the sound fading into silence, when you have stopped singing. But I don't know what an "Abeeyo" is.
Schlusstöne:
Plural for ending sound.
hereinklingen:
It means that the two voices can overlap in the case of slower chanted "abbeyo" phrases.
Hope this helps you. But be aware.....
I don't know much about music.