What is being recalled in the poem "Piano" by D. H. Lawrence?

In this poem, the speaker recalls his childhood. In the present time, the speaker is listening to a singer accompanied by piano music. In the present moment, he is an adult man. The song evokes memories of his childhood because it reminds him of his mother singing at the piano. He is mentally transported back to his experiences as a child sitting under the piano while his mother (presumably) played and sang. 

Note that in...

In this poem, the speaker recalls his childhood. In the present time, the speaker is listening to a singer accompanied by piano music. In the present moment, he is an adult man. The song evokes memories of his childhood because it reminds him of his mother singing at the piano. He is mentally transported back to his experiences as a child sitting under the piano while his mother (presumably) played and sang. 

Note that in the second stanza, he is transported to this memory in spite of himself. As a rational adult, he seems to resist such a romantic, sentimentalized longing for childhood. As the speaker moves into the third stanza, he says that by this time, the song of the present time is not so relevant anymore. And this is because he has been fully transported (emotionally, mentally) back to his childhood. His emotions have overrun and conquered his adult, rational mind and sent him back to the past. That's why he says his "manhood is cast . . . " Initially, he thinks it is irrational to be conjured to this overly sentimental memory, but the emotion is too much and overrides this resistance: 

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.