Posting for the sound
Why is there no ending after أُمّ
I would say it's because the ending depends on the role of أُمّ in the sentence. Since this is not a full sentence, we cannot know the role of أُمّ (subject, direct object, indirect object...), so we cannot give an ending to the word.
But if أُمّ is subject, should we pronounce the ending namely the raf' (الرَفْع) ?