Possibly when module dependencies were calculated by depmod it rejected those modules that aren't owned by root - security risk. There's some info here: http://linux.about.com/library/cmd/blcmdl8_depmod.htm (just search for "not owned by root")
You might try changing all ownerships to root and then rerunning depmod. e.g. sudo chown -R 0.0 /lib/modules/2.4.26 sudo depmod -a
And then try modprobing the alsa modules.
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or is it possible to update kernel manually to 2.6
You could always try DSL-N rc4 (2.6.12) - the alsa modules are included already, but you will still need the alsa_mod extension from the DSL-N repository.
original here.