youtube-dl will give you information on what audio codecs YouTube is offering:
Code:
$ youtube-dl -F https://www.youtube.com/watch?v=u2dIeM8Tf-E | grep -v 'video only'
[youtube] u2dIeM8Tf-E: Downloading webpage
[youtube] u2dIeM8Tf-E: Downloading video info webpage
[info] Available formats for u2dIeM8Tf-E:
format code extension resolution note
249 webm audio only DASH audio 63k , opus @ 50k, 7.78MiB
250 webm audio only DASH audio 81k , opus @ 70k, 10.07MiB
140 m4a audio only DASH audio 130k , m4a_dash container, [email protected], 19.66MiB
171 webm audio only DASH audio 134k , [email protected], 15.49MiB
251 webm audio only DASH audio 159k , opus @
160
k, 18.88MiB
17 3gp 176x144 small 81k , mp4v.20.3, [email protected] 24k (22050Hz), 12.48MiB
36 3gp 320x180 small 225k , mp4v.20.3, mp4a.40.2 (22050Hz), 34.58MiB
43 webm 640x360 medium , vp8.0, [email protected], 95.22MiB
18 mp4 640x360 medium 431k , avc1.42001E, [email protected] 96k (44100Hz), 66.11MiB
22 mp4 1280x720 hd720 1143k , avc1.64001F, [email protected] (44100Hz) (best)
Note the 3gp targets.
I haven't checked this but I assume everything is encoded in CBR?