箫

Top bamboo radical, the compressed top form of 竹 - two stylized bamboo stalks side by side. It indexes 箫 in the bamboo family: 笛 flute, 笙 reed-pipe, 笔 brush, 笑 laugh. The xiao 箫 is a vertical end-blown flute cut from a single length of bamboo, so the radical tells you what the instrument is made of.

Bottom 肃 supplies the sound through historical drift: sù → xiāo, attested in 萧 (mugwort, also a surname), 潇 (waterfall sound), 啸 (to whistle). Faint semantic flavor fits beautifully too - the xiao 箫 is the instrument of solemn court music and Daoist hermits, its breathy tone associated with quiet dignity, exactly the 'solemn, hushed' core of 肃.