Musical Mischief is one of the events in AdVenture Ages and is the fifth event ever released. The first 7 ranks are very easy, and then it comes down to waiting for population for a while. However, later in the event, Music Stand missions, population missions, and ramping up new generators takes a very long time, while Baton and Sheet Music missions usually complete the moment they show up. This event is currently the hardest in the game to complete, requiring buying the Rank 19 bundle, spending thousands of gems worth of warps, and possibly buying EXP as well, to be able to finish Rank 24. If you plan to buy the Rank 19 bundle, it is highly recommended to do so at Rank 20, as that will give you a high chance at getting lots of the Vinyl Records production rare.
The changes were posted on the Mission Tracker. One change is that the population rate has been halved, so it's expected to be more difficult next time without further changes. The max level of all heroes has also been increased by 1, to prevent getting lots of extra EXP from maxing cards. Some mission requirements were reduced a bit, but not by enough to make much impact.