Tried to mix down a song in Sonar Platinum today.
After the tweaking was done I did a mixdown, and to get maximum soundquality I enabeled  the 64bit engine in the export window. I used audible bounce so I could hear what was going on.
After a couple of bars I heard that something was wrong. I sounded like the bassguitar was fading down and almost disapeared in the  mix. I stopped the bounce process and checked if there was some level automation that was wrong, but the bassguitar had no automation. 
I tried a new bounce but the same thing happened. I stopped and checked if there was some compressor settings that fooled me, but I cuoldn't find anything. The song played fine when I was playing it in Sonar, but as soon I tried to export, this strange thing happened.
Then I unchecked the 64bit engine and the export/bounce sounded exactly how it was supposed to.
 
Anyone else with similar experience?
 
Edit: I'm using the latest update

The 64-bit engine should not make any audible difference. What it does do is double RAM usage during the export, which might cause problems. Simple solution: refer to the first sentence.

Is the bass guitar something you tracked or is it from a sample library or softsynth?  If it's from a sample library or softsynth, maybe it doesn't play nicely with the 64 bit engine.  Maybe you'll get better results if you freeze the track first before mixdown.

Using the letter E you can bypass all effects. This might help you troubleshoot.

I had a similar thing happen to me. Only it seems to be a different track with each bounce. Snare gone, then the guitar is louder for no reason. In have spent days working to isolate what is going on. My solution is to set up a second computer and just dub it off like the old 2 track days. The mix plays fine but bounce is just not usable or trustworthy. I had similar problems back with 8.5.  Who knows what it is... converter drivers the DAW itself ( gasp I said it) or something else. With only forum help to go by, there is no accountability so..... welcome to the soul sucking repetition of the new home recording world.

talon1studio
Who knows what it is... converter drivers the DAW itself ( gasp I said it) or something else.

My money's on something else. Audio drivers are not involved in bouncing, and too many users are not experiencing this problem for it to be a general issue with SONAR by itself. Most likely it's project/plugin-specific as I would guess the OP's issue is as well.
 
I do a null test of the live signal on the Master bus against a phase-inverted bounce going straight to Main Outs as a matter of course with all my finished projects, and I'm always able to get them to null to near - if not total - silence, barring the random variations (deliberate or otherwise) that many plugins produce.
 
I will say, however, that I've gotten in the habit of leaving the 64-bit engine off until the final export because too many plugins have issues with it. And I can't honestly say I've ever been able to hear the difference unless there's an obvious problem.

For forum help to be of any use, we need data. "Spent days working to isolate what is going on" is the only data we have, and it's a possibility that spending days trying to isolate things without a strategy may have created all kinds of weird variations from the defaults. We don't know what version of SONAR you have, like whether you have the one with useful presets in the export menu. 
 
I bounce everything from individual tracks to entire compositions for a final two-track mix with no problems, however for you to arrive at that same place we need to know your interface, the drivers you're using, what your export menu settings are, etc. Bouncing is a simple and trouble-free operation but it's a combination lock...if each tumbler isn't set correctly, the lock won't open.
 
Hmmm...maybe "Taming the Wild Export/Bounce Menu" should be a future "Friday's Tip of the Week."

There is really not much more to say except every bounce has a different issue
First  the snare is missing. Next time the guitar has jumped 5 dB. And since I had the issue with 8.5 before I ever started using waves plugins- they are all I use - I find it difficult to think it would be them. I suppose I could remove them all to prove they are not the problem.  Has anyone had this issue and found a solution that worked for them? As I say, the playback is fine, it is only the bounce that is flaky. Freeze changes the mix as well. It is like any automation wrecks the mix- except for normal play back. Not a problem unless you actualy want to share the music, or provide the customer their product.
If this is not specific enough, please ask and I will try to accommodate as best I can..
 
Platinum
Windows 8.1  4GHz quad core intel processor Asus board 16 gigs Ram.MOTU 828x..

As for the bounce settings I have tried with and without the 64 bit engine. No change. I ran tests with fast, regular and audible. No consistant repeatable bounce was achieved. latest drivers on the motu. Its a real disappointment. 

talon1studio
As for the bounce settings I have tried with and without the 64 bit engine. No change. I ran tests with fast, regular and audible. No consistant repeatable bounce was achieved. latest drivers on the motu. Its a real disappointment. 

So it works sometimes, just not consistently?  Your issue is different from OP -- he, at least, can disable 64-bit engine and get it to work.  You've got something else going on... Probably better if you started your own thread, and provide your system specs and other pertinent details.