Are you allowed in this situation to use a warning sound signal?

In the video, you are driving along a priority street marked with sign D-1, and near the end a black car pulls out in front of you from the right, turning left across your lane. This is an immediate danger, so you are allowed to use the warning horn. The answer is: yes.

The Road Traffic Act allows you to use a sound or light signal when it is necessary to warn of danger, and in a built-up area it allows the horn only when it is necessary because of immediate danger. A driver who has not noticed you and is entering your path is exactly such a case. The horn is an addition to braking, not a replacement for it - first you slow down.

Legal basis: Art. 29 para. 1 and para. 2 point 2 of the Road Traffic Act (Prawo o ruchu drogowym)