I have found (at least on my computer) the origin of the problem. It's in sound/pci/hda/patch_analog.c, in the ad1986a_automic function (line 658 in the current lenny version). If I disable this function, using an ugly "return" at the beginning, it works fine. However, I haven't the required knowledge to go further (tweaking the function in order to make it do what it should do, without breaking sound recording on certain models).
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