Hi Paolo,
I have a bug report that I noticed on V3 and V4, and have now figured out what causes it: when I dial a phone number from the dialer, all system sounds (notifications, locking/unlocking, ringtone etc.) become very quiet.
I believe that it's normal for the Android system to decrease the volume of system sounds during phone calls, but it doesn't seem to reset the sound level back to the regular one after the phone call is done. I can't find any change in system volume settings before and after this occurs.
The sound volume goes back to normal after performing a soft reboot after the phone call, so it's probably a problem somewhere in userspace and not the kernel.
I'm going to try and reinstall V2 and see if that happens there too, since CyanogenMod seems to have changed the dialer/contacts and SMS apps between V2 and V3.
Cheers!
EDIT: bug is not present on V2.
EDIT #2: bug is present in V2 but in different form, it's not enough just to call a number and then hang up, you have to actually have a conversation. Soft reboot still fixes it.
EDIT #3: I'm going to do a fresh reinstall and see what happens since this has gotten weird. Does anyone else have this problem?
EDIT #4: After spending yesterday testing stuff, turns out it's caused by some weird combination of apps on my part. Sorry for the false alarm