Talk in Silent
Finally have a found the real reason. When Talk is set to:
Notification: System Bar;
When a message comes and the chat is in foreground, the volume is changed to level 3.
The program I've made, once started, poles the sound level each 5 seconds and when it's different a notification appears.
To this post I've attached the .apk, its source and a silent ringtone to overcome this bug.