sigh, groan, loud laughter
唶 centers on vocal expressions of strong emotion: sighing or groaning in distress, and loud laughter in amusement. The common thread is an audible, often involuntary, outburst from the mouth.
唶 combines 口 (mouth) with 昔, which likely contributes the sound. The exact historical development is uncertain, but the structure suggests a phono-semantic compound related to vocal sounds.
The mouth 口 on the left and 昔 (old times) on the right: imagine an old person sighing or groaning about the past, then suddenly bursting into loud laughter at a fond memory.
For シャ, picture a person sighing 'shah!' in exasperation, the sound escaping their mouth 口. The sharp 'shah' cues シャ.