I have the same problem with 1080p videos.
So after testing a buntch of programs, aTubecatcher shows me something interesting.
Apparently youtube splits the sources for 1080p. The audio and video apear as 2 things with different bitrate, even with video (in this example the video has only 1 bitrate, but with other videos that i tested they have multiple bitrates)
After download the file named VIDEO/MP4 only the VIDEO was download without sound