In Vanilla, readied Destruction spells—Firebolt, Lightning Bolt, and Fireball—stop playing their sounds as soon as the player sheathes them regardless of how long the spells were readied.
After enabling only USSEP, readied Destruction spells do not always stop playing their sounds as soon as the player sheathes them.
Tested with the following:
SSE v1.6.640
SKSE v2.2.3
USSEP v4.2.8