I have no idea what is causing your issue, but I can assure you your Audio Interface has nothing to do with it - IF you are bouncing offline. Are you?
I must add that this sounds very odd/weird/strange/new, I never heard of a project in which some instruments fail and others not. I would urge you to post your question over at http://www.logicprohelp.com/index.php?file=home
registration is free, and that forum has the possibility to attach project files and/or bounces to posts, so people can troubleshoot it for you. Going only by written descriptions in some cases just doesn't work.
Some info that is important:
1. Exact Logic version
2. Exact OS version
3. Mac model, amount of RAM, amount of disks (internal and/or external)
4: AMOUNT OF FREE SPACE LEFT on your startup disk. If this is too low (=less than 20% of the disks' capacity), you get all kinds of trouble, not just with Logic.
Also: are you saying that everything sounds okay when playing back in Logic? and that it is just the bounced file that sounds "off"?
Thanks for the revert ... I apologize , i was fiddling too much with the Bit rate which messed things up ... I finally bounced my track (Mp3@ 320kbps) and boy it sounds awesome ...!!
The one problem that I am facing is that I get a message (Memory Too Low) ... Now, I do use a lot of Au instruments which of course require a lot of power.. I don't notice spikes on the HD performance but CPU load goes all the way up (red spikes).. I have a 2.26 core 2 duo white macbook , upgraded the Ram to the max 4GB (DDR3 1066MHZ) and have around 50 GB (250GB 5400rpm HD) of free space ...
I'm wondering if I may have to upgrade to a faster Imac to continue working ...??
What you could do to squeeze every byte of RAM out of your Mac is to run Logic in 64 bit mode, it can access 500 to 800 MB more - IF you have no other programs running. However, since you have only 4 GB RAM chances are that soon everything will become very laggy. If it does, save, quit, reboot your Mac, wait two minutes so your Mac can 'settle'. Now, with a clean chunk of empty RAM, start Logic and the project.
I advise you to install a little utility on which you can monitor your RAM-use, such as Istat menus or Menumeters. That way you'll get some insights into how much RAM is used, and how much is available.
There are strategies to combat this, the simplest being to bounce big sampled instruments to audio and to purge the samples from the original track. Bouncing flexed tracks in place and deleting the original tracks is another option. I
t really depends on what it is that is eating all your RAM.