Where can I find good before and after sound samples for learning eqing, compression, mixing or even mastering?
Would be great if there are detailed explanations to each mixing action.
You can make your own. Take any single professionally recorded audio source you can find and mess with the EQ at random. This will be your "bad" and the actual recording will be your "good". EQing is really something you are best to learn by feel. Get some good audio tracks that are just an instrument or just a vocal or just speaking. If they are well done, they should sound pretty good to you. Then start adjusting the EQ and listen to how it distorts the sound.
The process for setting EQs is basically just the same thing in reverse. We start with a sound that doesn't sound quite right and apply the same amount of adjustment, but in the opposite direction. The exact amounts things are off are the result of the pickup pattern of the mic, the sound of the source and the environment the source is in. Some amount of artistic correction for what sounds "good" may also be factored in based on the sound of the source to emphasize certain parts of the sound.
If you want to be able to practice, simply export the audio after applying some alterations to the EQ, then open up a new project and try to get it back to close to the original by adjusting the EQ yourself by ear.
http://www.soundonsound.com/ has an archive of before and after files for a wide range of instruments and voices in their mix rescue section
There are also forums where persons give stems foe a mix and others uopload their mixes . You can learn a lot from that experience
There are a ton of great instructional videos on YouTube!