According to Google you can use quite a bit better audio than that in a 1080p video. That 128kbps is for mono audio!
It would seem to me one should convert the original lossless audio to the intended aac (384 kbps) directly. That way Youtube wouldn't need to transcode it for a second time, with ruinous consequences. Youtube accepts mp4, surprise surprise, so I don't think it ought to be too difficult to put it together. That being said, I have never made a video I'd consider uploading to Youtube, so my expertise is purely of the armchair variety.