<input x-webkit-speech> works for me, so you may want to just apply the attribute to all input elements regardless of type (and textarea elements for future support). I suspect that support for voice input on input types such as date may also be added eventually.

Edit: More at the HTML5 speech input proposal at https://docs.google.com/View?id=dcfg79pz_5dhnp23f5#y1f9 . It's apparent from this that you should also use the attribute on select elements too. I also can get x-webkit-speech working in current stable Chrome with an input type of speech.