I have 4 boards with an ALC 850 and it worked with every OS X installation I had, even in 4 and 6ch mode (but only stereo used). It also works with Logic 8.
Is the output greyed out in the prerefences pane, can you adjust the volume with f.e. additional keys on a multimedia keyboard? A friend also often said he had no sound - but he used the wrong jack  Have you checked this - it's the green (lime) one.
BTW: It isn't enough to exchange the kext, you have to use some more terminal commands. Replace the AC97 Audio kext and enter this in Terminal:
Code:
sudo -s (password)
chmod -R 755 /System/Library/Extensions && chown -R root:wheel /System/Library/Extensions
diskutil repairpermissions /
Quit Terminal, then delete /System/Library/Extensions.mkext (and Extensions.kextcache in Tiger) and reboot. Some friends of me also hadn't problems with their audio, this always worked. We mostly have the same boards, Asus A8N-SLI Premium, so it's not too much different than yours.
2 Opteron systems: OSx86 10.5.8, Andy's 9.8.0 kernel, Asus A8N-SLI Premium, Opteron 185 o'clocked @ 2 x 2,95 GHz (2nd system 2.6 GHz), ATI Radeon HD2600XT 256MB Dual-Monitor 2x HP L2035, 4 GB RAM, Griffin FireWave as main audio device, Marvell + nForce LAN, Asus U3S6 USB3/SATA6 card, 5,5 TB harddisk, Firewire 800 card, Apple Remote + eHome IR receiver, 2x Wacom serial graphics tablet, Canon Pixma iP4700, Logitech Internet Navigator wireless keyboard/mouse combination.
My Audio stuff: M-Audio Transit USB (default audio), M-Audio ProFire 610, M-Audio ProFire Lightbridge (34 channels) using Creamware A16 ADAT converter • MIDI: M-Audio Midiman 4x MIDI interface • Behringer Audio Mixers: Xenyx 1002, Xenyx 1002FX, Xenyx 1202FX, Eurorack UB1002FX, Eurorack MX1804FX, Eurorack MX262A • FX devices: Lexicon MPX100 DSP, Behringer DSP-1000 Virtualizer, Behringer MiniFEX 800 DSP, Behringer Multicom Pro MDX4400 compressor • RETRO: MSSIAH midi/sequencer/synthesizer cardridge for the C64 (Dual-SID), Steinberg M.S.I. MIDI Interface for C64
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