I asked a question and I will answer it myself.
It may not have been the best idea to use an old DVD drive case, but this form factor seemed convenient for both the power supply and the amplifier. The problem is that short-circuited circuits occur inside the closed case. And since the WiFi module is not shielded, a lot of interference and noise is induced on the analog parts of the board. The left channel op amp is closest to the Wi-Fi module and produces the most noise.
Empirically, I found a very simple and effective solution to the problem of WiFi noise and interference - this is antistatic packaging.
First, I prepared a paper template.
Then I cut out the shield from the antistatic bag.
I have noticed that interference is eliminated more effectively if the shield is located as close to the board as possible. Therefore, I glued it to the board with double-sided tape.
The shield on the bottom side of the board is also spectacular, despite the fact that the bottom of the case is located close to each other.
The WiFi module is also shielded.
The contact of the board with the metal case remained only at one point near the power connector.
After all the improvements came the long-awaited silence! And even with a close location of the WiFi antenna, noise does not appear. The amplifier has finally taken its place on the bookshelf.
