I'm using kde4.7.4 with alsa + phonon 4.6 + phonon-gstreamer, but kde fall into silence (i dont know when, may since i upgrade kde). when i try to test it in system sound dialogue, it played sound, but when i login kde it can't be played. how can i slove it? thanks!

Do you have any other applications which run on startup which could be occupying the sound card? (Skype for instance).

Another thing to check is that DMix is functioning properly - and that Pulseaudio isn't being used somehow.

there is no software using sound card when startup. i didn't use dmix because before notification sounds are available.
the problem seems to occur when i upgrade to kde 4.7, may the phonon is too old?

DMix is an ALSA feature required to allow multiple applications to use the sound card when no hardware mixing is available and no audio server such as Pulseaudio is in use.

Can you check that an "Audio" notification is enabled in System Settings > System and Application Notifications > Manage Notifications > Applications. Select "KDE Workspace" from the "Event sources" drop down. The "Login" notification should have an Audio notification of "KDE-Sys-Log-In-Short.ogg" configured.