taru-adjective, adverb taking the 'to' particle
echoing beautifully; beautiful sound
Archaic literary word describing a clear, beautiful ringing sound, especially of jewels or metals striking together, or figuratively of a melody. Used in classical Japanese texts.
In classical literature, the sound of jewels touching each other is sometimes described as 璆鏘.
In that poem, the old-fashioned phrase 璆鏘たる調べ (a beautifully echoing melody) appears.