The speaker is saying that it is in vain for the singer to try and entertain enthusiastically. The singing and music has taken the adult man down childhood memory lane. The speaker is reminiscing about childhood days and cannot help but weep.
The speaker cannot join the singer's clamorous enthusiasm. The speaker is in another time frame. He has revisited childhood when his mother played the piano and sang on a cold Sunday evening. The adult speaker is right back underneath the piano stool, near his mother's feet. He is remembering a precious time in his life as a child. It brings back such fond memories until he can only weep.
The speaker has traveled back down the road to his childhood. He has put aside his manhood and broken down, weeping for days gone by. The speaker's heart is wrenched at the moment. He can only long for the days when his mother's tender voice had a soothing effect on him. It is in vain for the present day singer to try and rally enthusiasm from her audience. It is in vain for the present day singer to try and entertain in a delightful manner. The speaker is already back in his childhood, sitting underneath the piano, at his mother's feet. The adult man wipes away his tears:
So now it is vain for the singer to burst into clamour
With the great black piano appassionato. The glamour
Of childish days is upon me, my manhood is cast
Down in the flood of remembrance, I weep like a child for the past.