Just demux the mp4 and keep the audio separate
Avidemux (http://avidemux.org) will extract the audio from a .MP4 file and write it to a WAV file.
As others have pointed out, the recommended method is to first determine what format the audio is in already within the MP4 file. If you can use it in that format, then just split it out to a file using Avidemux or similar.
However I tried using Mp4box gui, I see from the youtube.mp4 info that quality of the AAC file is only 96 or 100kbp/s. It's quite small than the others I got from Audioro Zune HD Converter. So the problem is, is that 320kbp/s getting from Audioro Zune HD Converter faked ?
According to Wikipedia, all YouTube videos have their audio in AAC format. Thus it should be trivial to extract the raw AAC. However, I forgot to mention above that you'll want to add a new MP4 container to the resulting raw AAC file with the [font= "Courier New"]-add[/font] option. I can't edit my above post--so here's what I hope is a slightly more descriptive, accurate, and hopefully useful list of steps:[font= "Courier New"]mp4box -info youtubefile.mp4[/font] = Get TrackID of the audio track[font= "Courier New"]mp4box -raw TrackID[/font] = Extract this track to a raw AAC file[font= "Courier New"]mp4box -add resultingfile.aac yournewfile.m4a[/font] = Add to this raw file an MP4 container (needed by many apps, including iTunes which 'prefers' the file extension m4a for audio-only files)
You could try MP4Box. Check its documentation to be sure, but IIRC you'll need to run -info to determine the ID of the audio track, then run -extract with that ID to dump it to a new file. There are probably GUI programs (or frontends for MP4Box) that can also do this, if you prefer that; have a search.As said already there's no point converting to MP3. You'll just lose quality and space. This applies to any format, even lossless ones (though in this case you just lose space).
...if I understand the issues with lossy to lossy conversion correctly.
lossless to lossy to lossy, a lossy to lossless to lossy,
For the trillionth time, there is no such thing as a direct conversion between audio formats. The source file is always decompressed to PCM before being fed into the encoder of the destination file. What seems to confuse some people and lead to questions like yours is that some applications hide this fact.
Interesting...with the .mp4 extension, iTunes will display the correct bitrate of the "-single" file (as well as reporting a video track with resolution of 0x0), but now the sample rate is "Unknown", and the "Sample Rate" field doesn't even show up in the "Get Info" dialog box...still plays correctly, though.Strange things are afoot at the Circle K...