This was only tested using "pure"-piano music and will help you find the correct notes being played. It will probably be way more difficult or even infeasible for music with more instruments, and this method cannot help you with rhythm and key-signatures.

Programs:

• MuseScore
• Audacity
• MPV
• YT-DLP

Download music using yt-dlp:

> yt-dlp -x "https://youtu.be/iGN-Xw1BSp4"

Using MuseScore, simple melodies and single notes can be mostly transcribed using trial and error by ear. MPV can help with this process:

• Speed up / slow down: [ or ]
• Mark and loop section: l
  • First two presses are used for loop markers. Section is looped automatically.
  • Press l a third time to clear the loop section

For more difficult sections (e.g. chords), analyse the audio section using Audacity: Open the file in Audacity and switch to spectogram view using the drop-down of the imported track and then Spectogram.

Select the beginning of the note / chord being played. The selection should roughly last at least >=0.20s for best results. The spectogram view should help you show when notes are being played.

Now let Audacity perform a spectrum analysis using Analyze -> Plot Spectrum.... A new window will pop up:

• Use the mouse to hover over the graph.
• Peak shows you the most prevalent note (and octave) being played close to your cursor.
• Upping Size will yield more precise results, but requires longer audio samples. 0.20s of audio are good enough for Size = 8192, which yields pretty good results on my end, but experiment if necessary.
• I set Function = Hann window and Axis = Log frequency, but it shouldn't matter too much.

Now the chord can be mostly extracted from the spectrum analysis by examining and reading each peak. Enter the notes into MuseScore and compare. Use your own judgement to decide which peaks should be ignored from the graph, as previously played notes, reverb, other strings resonating and some faulty / inaccurate peaks can make this process more difficult.

All in all this method can give you a "set of candidates" to choose from, which personally is incredibly helpful and easier than trying to decipher chords by ear. The rhythm is, personally speaking, not that difficult to figure out most of the time (given that you have used sheet music before). I personally don't really care much about the key-signature, so this method is sufficient for me.