headphones almost muted for mono audio

[Halbarad]

After backing up my system with Clonezilla Live, the audio volume for my headphones is very low (sounds are barely audible) but only for Skype and for mono sound files (WAV, MP3, OGG, etc.). Other sound files are played (for example by Audacious) at the right audio level.
Some relevant facts:
--The headphones worked well before the backup
--The audio volume for stereo sound files is ok in the earphones
--If I plug out the earphones, the speakers' volume is OK both for mono and for stereo files
--If I convert stereo sound files to mono with Audacity they "turn bad" and the headphones volume is now very low
--All the volume controls in Alsamixer and in Pulseaudio control are ok (top in Alsamixer and 100% in Pulseaudio pavucontrol)
--At max volume (about 150% in pavucontrol), the mono files are a bit louder but still muffled and much distorted in the headphones -- stereo files are deafening
--I have restored a complete image of the system taken 8 months ago but I still have the problem (which surely wasn't there even three days ago)
-----Therefore I guess that the problem has to do with the hardware or the firmware; perhaps a firmware switch or something
-----If some software switched it on, some other software can also switch it off.
--PC: Asus Z53H
--Sound card: Intel Corporation NM10/ICH7 Family High Definition Audio Controller (rev 02)
I made many more trials but most of them were irrelevant, in the light of the latest findings.
[NB: I submitted a previous version of this post in another thread but I have found out that much of the info I provided there is irrelevant or misleading and even the description of the problem is different]
If you have any idea, or if this strange behaviour triggers recollections about another thread, please tell me...

Sorry, complete moron here. I changed the earphones and everything worked. Shame.