Hi Ioan,
the squaks are easily to remeber - the numbers rise with the intensity of the emergency.
7500 - radio failure - bad but nothing to worry about to much, except in IMC, but there you should know your procedures anyway
7600 - hijack - more distracting than a coms-failure, isn´t it? But the bird is still flying.
7700 - Emegency, you´re ******* going down or very close to, might skid over the runwy without brakes or with skids with high speed, you´re on fire and stuff like that.
Bad news and air traffic should look on the radar either to know, where you´re going down and/or is shuffeling other traffic away, so that you get all the assisstance, i.e. direkt approach and so on, to give you the chance to handle the emergency succesfully.
So easy to remember, from a little problem to the !!!! 7500 to 7700 ;-)
Greetings Flying Bull