5.1 surround sound with NVidia4

Hi all,

I have working sound system, but I hear only two out of my 5.1 channels.
Any idea how to make them all working? Any links related to the topic
are also welcome. I googled around, but I couldn't find a solution. I
hear sound from the front left, right and subwoofer speakers during mp3
playing with Rhythmbox, and I am able to hear sound from the backward
speakers only by unmuting "Duplicate" in alsamixer, but that's not what
I want. I never managed to hear a sound from the center speaker.

I have a NVidia4 chipset based board with a Realtek ALC850 8-channel
codec on it.
I have debian lenny/sid 2.6.24-1-amd64 SMP with:

lspci -v:
    00:0d.0 Multimedia audio controller: nVidia Corporation CK804 AC'97
Audio Controller (rev a2)
    Subsystem: ASUSTeK Computer Inc. Device 812a
    Flags: bus master, 66MHz, fast devsel, latency 0, IRQ 22
    I/O ports at b400 [size=256]
    I/O ports at b000 [size=256]
    Memory at d7dfd000 (32-bit, non-prefetchable) [size=4K]
    Capabilities: [44] Power Management version 2
    Kernel driver in use: Intel ICH
    Kernel modules: snd-intel8x0

lsmod | grep snd:
    snd_mpu401             13992  0
    snd_mpu401_uart        13440  1 snd_mpu401
    snd_seq_dummy           8580  0
    snd_seq_oss            36736  0
    snd_intel8x0           41000  3
    snd_seq_midi           13376  0
    snd_seq_midi_event     12544  2 snd_seq_oss,snd_seq_midi
    snd_ac97_codec        118232  1 snd_intel8x0
    snd_seq                58752  6
snd_seq_dummy,snd_seq_oss,snd_seq_midi,snd_seq_midi_event
    ac97_bus                6912  1 snd_ac97_codec
    snd_pcm_oss            46624  0
    snd_mixer_oss          21376  1 snd_pcm_oss
    snd_rawmidi            30624  2 snd_mpu401_uart,snd_seq_midi
    snd_pcm                89352  4 snd_intel8x0,snd_ac97_codec,snd_pcm_oss
    snd_seq_device         12948  5
snd_seq_dummy,snd_seq_oss,snd_seq_midi,snd_seq,snd_rawmidi
    snd_timer              28552  3 snd_seq,snd_pcm
    snd                    65896  16    
snd_mpu401,snd_mpu401_uart,snd_seq_oss,snd_intel8x0,snd_ac97_codec,snd_seq,snd_pcm_oss,snd_mixer_oss,snd_rawmidi,snd_pcm,snd_seq_device,snd_timer
    soundcore              13216  1 snd
    snd_page_alloc         15248  2 snd_intel8x0,snd_pcm

cat /proc/asound/cards
 0 [CK804          ]: NFORCE - NVidia CK804
                      NVidia CK804 with ALC850 at irq 22
 1 [UART           ]: MPU-401 UART - MPU-401 UART
                      MPU-401 UART at 0x330, irq 5

    Cheers,
    Ivan


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