The even quavers sound suppressed and the odd ones sound pronounced from bars 5-8. How is this effect being produced? Can anyone help me with that?
There's no magic involved. You just play the "off beat" notes softer than the others!
Your example is fairly easy, because 5ths and 6ths are is a comfortable separation between the top and bottom parts. This Chopin etude is harder, because the parts are closer together, you have to play the accompaniment notes with your fingers as well as your thumb, and there are simultaneous "loud" and "soft" notes played with one hand:
Position your right hand by twisting more towards your pinky finger in the same sort of motion you would use to twist a door knob. This position causes there to be more distance between your thumb and the keys, which means that it's harder for your thumb to bang the keys. Your goal is to have less volume on the notes played by your thumb. The thumb has a tendency to hit the keys harder since it has less travel distance than other fingers. You'll notice that there is more pressure on your pinky finger than your thumb as a result and the melody will be much louder than the inner notes.
Try to think of the melody as melody, and of the supporting notes as supporting notes. Focus on the melody as the important thing, and treat the supporting notes as a softer, secondary structure. Play the melody completely by itself a few times until you're happy with how it sounds, and then just fill in the softer in-between notes without giving them too much weight.
"Press" on the melody note, give it more weight, both metaphorically and literally. There's a hint in the score that you should be doing this: you're asked to hold it for a full quarter-note, as opposed to the supporting notes which are just eighth notes. However, don't take this too mathematically, interpret it instead as a heavier, more important note, standing out, and the supporting notes below as softer, less important notes, that do not need to be pressed / held as much as the main melody.
The upper notes are (or share) quarter notes, the lower notes are mere eighths.
So you continue holding the upper notes while playing the lower notes. This works perfectly even on non-percussive keyboards like an organ or accordion.
Practice this at first. Once it works well, add a bit more stress on the "double" notes so that the slower line stands out reasonably well.