I have this problem all of the time, seeing as I use a Legendary stopwatch which is upgraded to boost crit time by ~10 seconds (it's only upgraded 3 times, aka, 1 mod is active). When the fight is completed while the crits/crit noise (the static-like sound that is made when in crit mode), this noise continues outside of battle. The only way to get rid of it is to restart the app, or to win another fight in which you activated crit mode, but let it end before completing the fight.
This problem isn't totally gamebreaking or anything, but hey, it starts to drive you nuts. It's kind of similar to Chinese water torture. XD
I'd suggest automatically turning off crit noise at the end of each battle once the enemy is defeated or once the played has navigated back to the map. Thanks!