Start splitting the ALSA and OSS code. The --v4l2-adev device will be tried first as an ALSA device, and then as OSS upon failure. If no audio device is specified, "default" is used for ALSA, "/dev/dsp" for OSS. You can disable/activate usage of ALSA and OSS using the --v4l2-audio-method option (see vlc -p v4l2 --advanced --help-verbose).
Next step will be to split the module in different source files, the current code is quite a mess.
Also fix compilation of the module if HAVE_ALSA isn't defined.