Inconsistent ReverbZone / AudioSource.volume behavior

I have a weird problem with AudioSource. There are 5-10 active sources in the scene and a Reverb Zone. One of the AudioSource is a 2D looped rain, the other is 3D looped thunderstorm affected by Reverb Zone, the rest is 3D looped engine sound and casual event sounds.

Some of the sounds have different levels of volume, for example, the rain and storm has both 1.0 and the engine has 0.17. Also, the event sounds have different volume levels based, for example, on collision's relative velocity.

The strange thing is that sometimes the rain is too quiet or the engine is almost impossible to hear. It's still here, as I can change the volume of rain and actually hear the difference, but it's played WAY too quietly.

Sometimes, the dry event sounds are not played, but they affect the reverb (which is pretty weird), so I can sometimes hear only the reverb (wet) part of the sample.

The even more strange situation was when the engine loop volume somehow coupled with rain volume, so changing the volume of engine (in the inspector during the play mode) affected the volume of rain.

I doubt it's caused by my scripts, because I only change the pitch of the engine and play some clips randomly with random volume (each has its own "One shot audio" source), also the problem sometimes appears in other, less sophisticated scenes as well.

It's very rare and hard to reproduce. It just happens randomly. For most of the time everything seems to be working OK. It behaves like messed up script priorities, but I don't really use anything besides basic audio functionality in my scripts, moreover, I turned them off where possible to test it out. Also, why does the Reverb Zone catch the full clip with correct volume, while it's not hearable directly?

I think that it might be Unity's bug, but I might be wrong, because it's really hard to reproduce, especially in simple scenes. Did anyone have similar problem? Any ideas how to solve it?

Is there any way to use a third party audio library if the problem remained unsolved?

EDIT: I found someone with the same problem: http://forum.unity3d.com/threads/persistent-bug-audio-reverb-zone.191811/

It seems that the whole weird volume behavior is a ReverbZone bug, which mutes the dry sound from time to time. However, I know that Unity 5 introduced a brand new audio system, so I hope I'll never encounter it again (didn't happen so far ;) ).