INTRODUCTION
I'm fighting this beast for years now. With recent LMMS releases it has been worse than ever. Almost every project I start gets infected by this weird "virus" and I can't play the song or render it.

This is a very serious problem, as it makes the whole program unusable. No need to mention the frustration it causes.

I feel in need to lead an investigation to find the root of the problem and solve this once and for all. And I need your help.

MY SETUP
I am running KX Studio 14.04b 64-bit on a ASUS-K52J laptop with Core i5 processor and 8GB of RAM.
LMMS 1.0.3 from KX Studio repositories.

WHAT IS IT?
The bug makes L and/or R channel of LMMS master output mute with huge DC offset.
This "loud silence" has a very special property: it'll mute your whole computer, and no filters can remove it (AFAIK). The problem seems to attack projects that reach a certain level of development (small projects doesn't seem to be affected).

How it looks like

The oscilloscope should show you a massive DC offset. But sometimes it just looks silent, regardless of the instruments playing and signal visible in the mixer strips.

Sometimes the ILS (Infintely Loud Silence) mutes all mixer channels, sometimes none, but master, sometimes just a few of them, as can be seen below:

Here's a screenshot form Audacity, viewing a rendered project affected by ILS:

How it sounds like
Here's a sample from the rendered project seen on the screenshot above:
http://www.freesound.org/people/unfa/sounds/243833/

Here's a video I recorded to explain the problem (I sent it to the mailing list before):
http://youtu.be/YgXWC91DWVc

FACTS

1. The problems seems to start after the project is rendered for the first time. Before the first rendering it hardly ever occurs;
2. After rendering affected project to WAV I get an nearly empty file with huge empty space inside (checked with hexdump);
3. The problems is very random and hard to catch - sometimes projects that were not usable a while ago load and play fine, sometimes projects that loaded and played fine suddenly get affected and are unusable;
4. I believe ILS strikes less frequently on fresh systems, without ton of stuff installed (and without ton of updates installed). However I've installed a fresh system last week and now I've got the ILS mayhem again. However I let it update all the way, so maybe it won't happen if I don't upgrade the fresh system;
5. When you're running LMMS affected by ILS and other programs that have audio output too - the other programs will also have L and/or R channels mute. Looks like wherever Infinitely Loud Silence enters a signal chain - all other signals mixed there are removed and the sum of the equation is just ILS alone, a huge DC offset. I know, it's crazy;
6. In rendered projects - ILS frequently kicks in in the middle of the song (see the Audacity screenshot above), with the rest of it just ILS itself;
7. 16 or 24-bit sound files can store and restore some part of ILS, it's not actually ILS, it's more like a massive DC offset that leads to distortion of whatever is played through the same signal chain, but it's not muting other sources completely anymore;
8. The problem is removed when loading new project in LMMS - whatever is performed during that process - kills the bug (which can re-appear immediately after the loading window disappears but no earlier!);
9. One instance of LMMS can trigger this problem to another running in parrarel. I had two instanced open with different projects loaded, I started rendering with one instance, the second one showed the DC offset of one channel in the ossciloscope, and started pouring bad clicks in the left master channel. The DC offset didn't disappear after the rendering finished, but the clicks stopped. However when I switch between the two LMMS windows, a bunch of clicks sound in my left headphone. They also shot series at quasi-random - seems to be somehow related to CPU usage - when I scroll a webpage with mousewheel, the clicks appear.
10. JACK Bitmeter shows that the LMMS outputs NaN samples (Not a Number):
    
    The above image shows a screenshot of two Bitmeter's instances connected to Left and Right LMMS master JACK output ports.

PROJECT FILES
Here are some projects that are "infected" - the problem appeared in them at least a few times.
Example projects.zip

One project, two versions - version 10 is afected by the bug, while 08 is not. Maybe we can track down something that has changed in the project file? One thing is I used here suspected zita-rev LADSPA plugin (in both project versions). I didn't render the project, but it broke after a few hours of working on it.

That's all I know for now. Please let me know if you've ever had this issue, or if there's anything I can do to help hunting it down. You can contact me personally via e-mail:
unfa@unfamusic.com

-unfa