Music + SFX + Dialogue + ADR + Background Noise + Foreground Noise = Sound Mixing.
Slumdog Millionaire, The Bourne Ultimatum, Dreamgirls, King Kong, Ray, The Return of the King, Chicago, Black Hawk Down, and Gladiator.
Songs factor into Slumdog, Dreamgirls, Ray, and Chicago heavily and all but Slumdog were not nominated for SFX. I don't think this precedent for victory plays heavily with these nominees.
I don't think Inglourious Basterds has much a chance of winning, nor Transformers as it was not nominated for Best Sound Effects and Lord Knows would stand a better chance of winning there. Star Trek has a better chance because it is mixing as many "things" as Transformers but with more clarity. But I think this is between Avatar and The Hurt Locker.
I think the closest precedent set of these is King Kong to Avatar. A pulsing score, tons of Sound Effects, moments of quiet, and lots of hardcore action sequences. The Hurt Locker finds precedents in The Bourne Ultimatum and Black Hawk Down. Ultimately, those two films had much larger budgets. Budget certainly isn't a deal-breaker, as The Hurt Locker cost about as much as Slumdog Millionaire, but I think this is probably Avatar's to win.
Avatar -- 45%
The Hurt Locker -- 30%
Inglourious Basterds -- 5%
Star Trek -- 15%
Transformers: Revenge of the Fallen -- 5%
"If you are marching with white nationalists, you are by definition not a very nice person. If Malala Yousafzai had taken part in that rally, you'd have to say 'Okay, I guess Malala sucks now.'" ~ John Oliver