I really don't know when this started to be supported but here's a video example with Dolby Digital Plus audio, Dolby Digital should also be supported of course but I still haven't found a video with it, please share one if you know of any.
AAC-LC 5.1 was already supported by YouTube but it seems to be available only on the Shield and select smart TVs.
The same video plays in DD if your TV doesn't support DD+