The song "comically makes it cringe-ably clear that Kristine is tone deaf while her husband (Al) helps her through it".
Kristine is the ditzy tone-deaf hopeful who is cleverly interjected by her husband Al in her "solo.
[5] The Real Chrisparkle commented: "When Bad Wolf and I were talking about the show beforehand, we both agreed that “Sing” is probably our least favourite number, because of its potential to irritate; just slightly.
You need the verbal dexterity of a Gilbert and Sullivan patter song, coupled with immaculate comedy timing and, from Kristine, the ability to sing credibly off-key.
[7] The Rider News noted "“Sing!” makes for a great contrast in the play, breaking the tension and adding in quirky humor".