So today I had some spare time and as a first test I downgraded all pulseaudio packages to previous version - 0.9.15-1maemo44+0m5+0cssu1. It seems that it has cured the notification sounds problem - when I get a call always my ringtone plays (no missed calls also) and also the alarm clock ringed instead of being silent. I'll continue to observe and test some more time of course. Hope this information can help to find the cause of the problem... And my sincerest thanks to CSSU developers for keeping our N900s alive