The 1080 p version is available without sound, last i checked. download a lower res version, then mux the audio with the 1080 p video.
for example this youtube video
change res to 1080 p with the star button followed by dropdown box.
In video download helper wait for only 1 flv ( without res mentioned) to be available as shown
1080pillustration..jpg
download that and change the extension to mp4, that is the 1080 p video. download one of the lower res vids in mp4 format such as the 720 p video. Using a GUI for mp4box such as YAMB, combine the video from the 1080 p file with the audio from the lower res file.
This is the resultant 1080 p file with proper audio

Download from mediafire-75.86 MB
edit: moved to help forum for more answers

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