Some translations set poetry farther to the right than the rest of the text to show that it is poetry. The ULB does this with the poetry in 9:1-12, 17-24.
The prophecy revealed to Jeremiah is recorded in 8:5-9:12. Despite covering more than one chapter, this should be seen as one unit. (See: [[rc://en/tw/dict/bible/kt/prophet]] and [[rc://en/tw/dict/bible/kt/reveal]])
This chapter pictures Jerusalem as having a funeral with its typical songs. This is an image used to describe their destruction. It is a time of great sadness.
Circumcision is used here in an unusual way. Since circumcision was intended to be a display of faith, the author uses the term to differentiate between those who have faith in Yahweh and those who don't. This is a type of metaphor. Those who have a circumcised heart have faith in Yahweh while those with an uncircumcised heart do not have faith in Yahweh, regardless of whether they are physically circumcised. (See: [[rc://en/tw/dict/bible/kt/circumcise]] and [[rc://en/tw/dict/bible/kt/faith]] and [[rc://en/ta/man/jit/figs-metaphor]])