OK, you have two problems.
1. Those MP3 download options listed on mygomp3.com imply that YouTube is hosting those MP3 stream options - and this is
not true.
It's important to understand that YouTube videos are natively in MP4 or WebM container formats, with video codecs H.264 and/or VP9, and
audio codecs AAC and/or Opus.
So when you see MP3 download options it means that the download website is re-encoding (transcoding) the original AAC or Opus audio files hosted by YouTube.
At this point it's important to understand the basic principles of compression; AAC, Opus, and MP3 are all lossy codecs. Lossy compression is OK ... provided you only use it
once in your audio production chain. In your situation you are sourcing an audio file which was AAC or Opus in its original state, but your download website is re-encoding it to MP3. That's bad - the website is doing a lossy encoding of a lossy encoding. Sure, you can specify the higher encode-rate of 320kbps, but that simply means the MP3 encode will lose
less information than one of the lower data-rate MP3 encodings.
2. Next you are importing this downloaded MP3 file into iTunes, with iTunes' import settings configured for AAC compression - so iTunes is doing a lossy encoding of a lossy encoding of a lossy encoding. Ouch! 3 stages of lossy compression. No wonder it doesn't sound good.
This second problem could be overcome by changing iTunes import settings to MP3 - then the import would proceed without further degradation ... however the first problem remains - a re-compression from AAC/Opus to MP3.
The full and proper solution is to download the YouTube audio file in one of its native states - in your case, preferably AAC. And for this all you need to do is use a better download website! One that automatically determines all possible file download options available for any YouTube site, then offer all of those options. The good one I know of is
TubeRipper
https://tuberipper.com/24/
I have attached a screenshot which shows the different audio file download choices (for Nirvana's Smells Like Teen Spirit on YouTube). I will briefly explain the options;
- m4a, MP4, ogg, and WebM - these are all container formats for the available AAC and/or Opus streams. ogg and WebM are somewhat non-mainstream, and certainly will not be recognised by iTunes to import from. m4a is a simplified form of MP4 - designed to contain audio-only, instead of video+audio.
So m4a is the best option for most people, and especially for you since iTunes will import it without any form of transcoding, assuming iTunes is configured for AAC encoding.
- FLAC is lossless compression, and WAV is fully uncompressed, so these would be OK - TubeRipper will uncompress the AAC or Opus streams and store this data without further degradation ... however it's a waste of file space, since the file size will increase massively, for no increase in audio quality. You can't put back audio data which was lost during the original compression - that's a basic principle of lossy compression.
- MP3 is the worst possible choice, since TubeRipper will re-compress the AAC/Opus stream into MP3, as mentioned earlier. The only reason you would choose MP3 is if you have a device or system which can only recognise MP3, and not AAC.
