In his Inaugural Address Lincoln said that the mystic chords of memory will yet swell the chorus of the Union to what memory does he refer to

In his Inaugural Lincoln Address to "common commitment to a moral truth that makes us people" by calling it "the mystic chords of memory".

Explanation:

The reason behind this is that lincoln context was very difficult, there was a very wide slave abuse and he was a supporter of slavery abolishment. Also, many states wanted to leave the federation because they wanted to pursue their schedules leaving behind the objectives of the federation. including human rights, justice, and commitment to freedom. Therefore, he mentioned the "mystic chords of memory" because the colonies fought against injustice, to get freedom and be treated properly. But people had forgotten.