The new sound for getting a correct answer...
Scared the &$*% out of me. They must have implemented it AS I was doing Duolingo, I had no idea what the sound was and I spent like 15 minutes checking my extra tabs like FB and Youtube trying to discover the origin of the sound.
That being said... I like it. :)
I enjoy the sound for failing too. Failing is way more fun now!
But, I don't know if this is because of the new sound effects in any way, I am encountering a bug (at least using Safari). I was practicing all skills, timed practice, and sometimes it would get to an audio question and the audio wouldn't play and the clock would appear to freeze (though it still counts down internally). Even if I clicked the play button, nothing happened, but I could play the slow audio. Once I did that and answered the question, the clock would start counting again, from wherever it ought to be after the time I took.
I think this should be introduced somehow. Maybe show a popup on the first session, like: "Hey, we've added new sounds, try them out! If you don't like them, you can switch them off in your profile settings." A more "personal" notification than a thread in Discussions feels better.
I think something like that could be done for all updates. Not everybody reads discussions and very often people fail to find the right announcement and post their questions in new threads.