You need to separate the argument array yourself:
var a = spawn('avconv', ['-v', 'quiet', '-i', 'pipe:0', '-vn', '-f', 's16le', '-acodec', 'pcm_s16le', 'pipe:1']);
The space delimitation you are used to from command line work is provided by your shell (bash, zsh...). The shell breaks up your command into argument using spaces and lets you say "I want this as a single argument" by adding quotes.