I've been seeing conflicting explanations for this character 昌. Some sources say it comprised of two suns 日, some sources say it comprises of sun and speak 日 meaning flourish, though I don't really see what connection it has to the sun. And then I've seen explanations that it's two 日 speaking mouths, being the original character for sing 唱 and sun is not a component. Which of these is the correct?
「昌」 (early morning call, e.g. to the day's labour) is comprised of semantic 「日」 (sun) on top of semantic 「口」 (mouth). Such calls would likely be rhythmic or melodious, similar to bugle calls, making the interpretation of 「昌」 as the original form of 「唱」 (to sing).
Flourish does not appear to have a direct connection to morning call, and should be regarded as a phonetic loan.