A user reported some crackling on their new iPad:
https://discourse.audulus.com/t/crackling-sound-on-ipad-pro-11/1371
Anyone else have an iPad Pro 11 and can confirm it’s happening for them too? I’ve heard it’s affecting other music apps, not just Audulus.
I have a 12.9 with crackle. From my understanding Apple is aware of the bug and the next update appears to have the fix.
I’ve got the 11" Pro and was also experiencing a lot of crackling in most music apps including Audulus. iOS 12.1.2 beta 1 has fixed the problem though, I’ve installed it a couple of days ago and haven’t heard a crackle since. This fix is also specifically mentioned in the release notes of 12.1.2 beta 1.
Apparently it had something to do with Core Audio utilizing the energy-efficient cores of the A12X instead of the high-performance ones, not completely sure though.
I think the public beta is out now, so you could install that (it works fine for me), or wait until the final release.
@Latzenpratz - see what @balph said above - it’s basically an iOS software issue and will be solved soon! 
With the beta update 12.1.2 the crackling is gone! Thanks!