"Um, excuse me."
あの has always struck me more as a general moment of hestation, while えっと is more like taking a moment to think. Adding あの to すみません makes it a little more hesitant, like you're hesitating to ask, so it softens the whole phrase. Perhaps sounds a little feminine, as females are expected to speak a little more politely than males?
すみません is the polite negative form of the verb 済む (sumu) which can mean both to be completed or to feel at ease. So the negative form would literally be "to not feel at ease/to feel guilt/to be unfinished" all roughly translating to the same feeling of 'excuse me' in the sense of needing to endlessly apologize for being a disturbance to others.