昌 is two suns stacked — brightness doubled — and lends both its sound and its energy. Put a mouth 口 beside that double brightness and you get singing: the voice at its most radiant.
唱 anchors two pillars of Chinese social life: 唱京剧 Beijing opera, where a single 唱 phrase can take years to master, and KTV, where no mastery is required at all. Refusing to sing at KTV takes more social skill than singing badly.
A mouth 口 shining like two suns 昌 — SINGING at full brightness.
K歌 'K-singing' (from karaoke) is so embedded that 唱K is a standard verb phrase — and booth-sized solo KTV pods in malls let you 唱K alone between errands.