Yeah, I reached out to them about getting the .ipa file but they said “I’m afraid that is not something we support. Sorry but the iLive peripherals and apps are completely discontinued.” It just seems odd that the OneMix app is not available but the MixPad and Tweak app appear to be.
If someone has the app loaded on a device using iOS 8 or earlier in theory they should be able to use iTunes 12 (I believe 12.6 or lower) to backup the app to their computer and will have the .ipa file. I don’t have a device with that iOS version to confirm but from what I have researched it should work.