are the headphones wired or wireless?
and virtually any video you watch on youtube, you see this delay? even if you pick a random video?
if you are using wireless headphones and the video is HD (full HD or any high bit rate video) there will be _SOME_ delay in the audio (lips will proceed the audio a couple of frames) due to the decoding of the audio stream. wired headphones and the in built speakers you wont see much if that at all unless the video (the source) was encoded wrong to begin with.
if you care to post up a link to a video you are watching on your youtube app, others can test and see if they notice the same issue.
when I watch a blinkfeed video for example, and listen to the in built speakers, i see no lag at all. even the youtube app on the videos I look at with the inbuilt speakers there is no lag. i use LG tone bt wireless headphones and I _will_ notice lag due to the encoding/decoding of the stream plus adding in the wireless aspect--this is why i asked about the headphones.
care to mention the phones you use?
its possible what you are experiencing is normal. some people won't notice this lag as much as others will.
if you think your device is defective, return it if you are able to do so. no need to go through all of this if its still in the return/exchange window.