The audio output on the MacBook Pro turns red. What to do?

Yesterday my MacBook’s audio output turned red and macOS started behaving strangely. The symptoms are as follows:

1. no sound from built-in speakers (when macOS is loaded);
2. there is no way to adjust the volume;
3. sound plays only through headphones or external speakers;
4. the headphone jack lights up red.

As it turned out, the matter is in the closure of the contacts in the headphone jack. And the solution is extremely simple – we take a toothpick and carefully touch the inner walls of the connector, after that everything should work fine. It helped me, so try it.