Describing the sound of 491 mandolins all playing the same song at the same time is almost impossible, though you can try:
A choir of extremely cheerful angels.
A thousand silver needles dipped in liquid sugar and dropped on a polished mirror.
A very melodic rain hitting a perfectly smooth lake.
(As one musician remarked to another afterwards, “I was afraid it would sound like a bunch of bumblebees trapped in a dryer.” Thankfully, he was wrong.)