I dont think the hum is the sound card. 

Ok but you should still test a few things as to remove the highlighted word!    1- Did you turn off phantom power? 
2- Did you unplug everything and then ONE at a TIME plug them in and make notes as to how much hum each component is producing? 

3- Do you get a hum from the mixer when you plug in an other   device, like a drum machine or keyboard, tape deck?? Microphone??
4-Does the hum change when you move components away from each other?? 
5-Does the hum go away if you add a ground lift to the AC plug? 

Another note that shouldn't make a difference, but to be aware of, those older Mackie mixers used to swap output pins 2 and 3 which puts it out of phase. At least the one I had was like that in 1994. 

that is interesting about the mackie.  I dont think this one is quite that old.  i would guess it is around 2003 or so. I have ground lifted everything.  but lots of good points there.  thank you very much

Well, the basic story is what you already knew you'd hear - You need to get an interface intended for DAW use, not a sound card intended for gaming use.  That's really all there is to it.

RB

Sorry to dig up an old thread, but I just installed a SB Audigy RX into my new computer yesterday, and tried using it with Sonar X3, so this seemed like a relevant place to ask my question.... 
 
First of all, regarding what the OP said about 16 bit, I experienced the same thing.  The card says "24 Bit", but when I opened Sonar, it said it had to switch to 16 bit since 24 bit isn't supported.  At least now, after reading this thread, I know why.  For my purposes, honestly, 16 bit is fine.  I'm not a pro, this is purely a hobby to pass the time, and I do entirely MIDI stuff, using softsynths, so I'm not trying to record external audio or anything. 
 
However, the problem that I AM having is that I keep getting a crackling sound during playback.  It comes and goes, but it's there.  Anyone know what might be causing that?  (I know the obvious answer is, "Your cheap sound card is causing it," and maybe that's true, but I've ALWAYS used cheap SB cards, and never had any crackling before.  In the past, I worked with Sonar 4 and Sonar 6.  This is my first time using X3, so I'm wondering if maybe there's some difference I'm not aware of in terms of settings.) 
 
 
 

How many soft synths?
 
What are your buffer settings (in the SB card)?
 
What is SONAR's Round Trip Latency showing? (Preferences->Driver)

It's crackling because it can't keep up... Depending on the chipset used,and if you are technically inclined, Google kxproject drivers

but I've ALWAYS used cheap SB cards, and never had any crackling before

 
Modern software places more demands on the hardware running it - this includes soundcards
 
Your crackling will not disappear until you take the only logical step.

It was a lifetime ago, but the old Audigy's had some ... problems.  I think there was a 16/24 split between in and out (playback of 24 bit material, not recording).  And the old emu engine used 48 KHz and resampled for 44.1, which produced artifacts.  either of those could be the problem, or maybe not.  This was over 10 years ago when I learned that stuff.
 
@

Ultimately it would be smart for one to look for a new audio card or device. One more suited to the needs of music production. 

In the old days some of the fixes for noise were turn off virus scanners, turn off all power management schemes, turn off Windows system sounds, turn off screen savers and wallpaper, ensure that you have the latest audio drivers for the interface and OS, etc.
With modern computers that have huge hard drives and boatloads of memory, I don't know if any of these are applicable, but you can try them.
I once had a Sound Blaster Extigy which advertised 24 bit but didn't discover that 24 bit only worked with the S/PDIF input until I hooked it up.