Strange sound problem

I have a somewhat strange problem with sound.

All other sound are (usually) ok, but sometimes when playing flash ie. from Youtube there is no sound. And sometimes video runs too fast also.

When running firefox from console while no sound:
:~> firefox
*** nss-shared-helper: Shared database disabled (set NSS_USE_SHARED_DB to enable).
Currently in use: 1 blocks containing 64.0 KiB bytes total.
Allocated during whole lifetime: 2766 blocks containing 8.7 MiB bytes total.
Sample cache size: 0 B
User name: xxx
Host Name: xxxxxxx
Server Name: pulseaudio
Server Version: 0.9.12
Default Sample Specification: s16le 2ch 44100Hz
Default Sink: auto_null
Default Source: alsa_input.pci_8086_293e_sound_card_0_alsa_capture_0
Cookie: 833bc07a
!!! [Hook] hook(): title not found

Then usually the cure is to run firefox as a root, but sometimes that does not help either. Just now after I had ran firefox as a root from console 3 times, sounds came back.

This kind of muting happens to all users sometimes, but whilst some might have lost audio, some other still might have sound.