Hello,
As of VLC 2.0.1, the intended channel mapping must be configured explicitly. By default, this is stereo because that is the safest and most common value. If you want surround output (or monophonic) you must edit the ALSA preferences in VLC (Tools -> Preferences -> All -> Audio -> Output modules -> ALSA!) and select the appropriate channel mapping and save. Then you can start playing.
Even though ALSA pseudo-devices (surrond51, surround71, etc) correspond in principle to a certain channel mapping, there is no reliable way from VLC code to determine the intended audio output channel mapping when using ALSA. The root of the problem is, the ALSA surround pseudo-device do not necessarily enforce their channel mapping, or not correctly. As an example, surround40 will accept 7.1 audio streams without but silently discards the extra 4 channels... Attempts in VLC 2.0.0 to automatically detect the correct channel mapping from the audio output device partly failed and caused much grief.
If you think this immensely sucks, then you are most welcome to switch over to PulseAudio where everything is done automatically.
N.B.: This is irrelevant for digital pass-through (S/PDIF). In that case, you need to enable the S/PDIF option as in previous versions and you do not need to care about the channel mapping.