Yes, it is. I had stressed this point in my previous explanation. There is, probably, an issue with the contact points but I cannot find the exact faulty part. Sound may start playing suddenly for a second in both headphones,if I push its audio plug in and out many times with a slightly different angle and not fully inserted. The same situation is found in a set of different headphones - the right "ear" is completely silent. As I already said a ring-tone speaker and the audio jack were replaced for new parts but that did not resolve the problem.
Today I flashed my Xperia x10i phone and returned it to a previous software version with my hope that it could eliminate the problem but no luck :(