My TV has sound lag if I use HDMI. Because of this, I always use HDMI video and RCA audio with my systems. When the Wii U launched, it didn't have the option to split it up like this and so if I used HDMI, the GamePad sound would be real-time and the TV sound would be delayed, i.e., out of sync. I went back to component video until the patch that let you use HDMI for video only (with both of these methods the GamePad sound was of course perfectly in sync).
So, your TV doing digital processing with the HDMI is probably delaying your sound. Try hooking the Wii U up with RCA sound and see what happens.
Do you have any other systems using HDMI? If so, they probably have delayed sound too. You just may have never noticed.