Almost always describes the sound of recitation or reading aloud - a near-fixed pairing with 读书声 or 上口. The fixed phrase 琅琅上口 means 'rolls easily off the tongue / catchy'.
culture
The character 琅 contains the 王 (jade) radical and originally referred to a type of fine stone or jade. The doubling 琅琅 imitates the bright, clear ringing sound of jade pieces striking - extended metaphorically to crisp recitation voices.
