It turned out that /dev/snd/* always exists even if there are no sound drivers loaded at all. Loading `snd` and `snd_timer` fixes that situation. It is probably fair to assume someone that wants to use sound also enables that in the NixOS configuration.
*** NixOS *** NixOS is a Linux distribution based on the purely functional package management system Nix. More information can be found at http://nixos.org/nixos and in the manual in doc/manual.