Is there any sun 日 in 昌? 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?
時期字體
字形
參考資料




商甲

甲185合集19924


戰國・燕古幣

108古幣文編


戰國・晉璽印

959古璽彙編


秦簡

日甲119睡虎地秦簡


篆

日部說文解字


楷






「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).
Later on, 「口」 was slightly shape-corrupted into 「甘」 (mouth 「口」 with a mark inside) or 「曰」 (mouth 「口」 with a mark above). The modern shape inherits 「曰」.
Flourish does not appear to have a direct connection to morning call, and should be regarded as a phonetic loan.

References:

季旭昇《說文新證》
小學堂字形演變
中國哲學書電子化計劃

宋・徐鉉等奉敕校定　漢・許慎《說文解字》


國學大師

郭沫若《甲骨文合集》
中國社會科學院考古研究所《殷周金文集成》