It did once, over 10 years ago (in 2008 or so, with the whole fmt=18 arguments etc), if your total video bitrate was below 320 kbps, it left the audio untouched ... I wonder if those videos from back then are still around and if yes, still have untouched audio.
Of course now it only has 96kbps aac, but it did have untouched mp3 audio back then.
And another update, I downloaded it back in 2013, where it still had 160kbps mp3 audio!
Note after formatting: As you can see from the video FPS, the trick was to keep the video bitrate as low as possible, by using a still image (or pad it with black + silence at the end). I'm not sure anymore if you had to also already use the same video codec as youtube did back then, i.e., if the video stayed untouched too. I never managed to upload a video like this, so I don't exactly know.
MediaInfo output:
General
Complete name : Luke Warner and Mat Lock - Deep Psychosis (Daniel Kandi's Cure mix) (HQ).flv
Format : Flash Video
File size : 12.7 MiB
Duration : 7 min 34 s
Overall bit rate mode : Variable
Overall bit rate : 234 kb/s
httphostheader : r13---sn-4g57knek.c.youtube.com
Video
Format : Sorenson Spark
Codec ID : 2
Duration : 7 min 34 s
Bit rate : 69.5 kb/s
Width : 480 pixels
Height : 360 pixels
Display aspect ratio : 4:3
Frame rate mode : Constant
Frame rate : 1.044 FPS
Bit depth : 8 bits
Bits/(Pixel\*Frame) : 0.385
Stream size : 3.76 MiB (30%)
Audio
Format : MPEG Audio
Format version : Version 1
Format profile : Layer 3
Format settings : Joint stereo / MS Stereo
Codec ID : 2
Codec ID/Hint : MP3
Duration : 7 min 34 s
Bit rate mode : Variable
Bit rate : 160 kb/s
Channel(s) : 2 channels
Sampling rate : 44.1 kHz
Compression mode : Lossy
Delay relative to video : -131 ms
Stream size : 8.68 MiB (68%)
Writing library : LAME3.97
