Literary/written register. Compare with 哭 (kū, to cry) which is plain everyday speech, and 抽泣 (chōuqì, to sob shudderingly) which emphasizes the convulsive shoulders. 啜泣 implies muffled, restrained weeping.
memory
啜 (chuò) originally means 'to sip' - picture sipping in tears one breath at a time. The 口 (mouth) radical anchors both 啜 and 泣.
