I'm also on Gentoo 64-bit with KDE and pulseaudio. Also have those same errors but videos have sound.
Ignore my comment if irrelevant : On arch linux x64 I had to install the packages : lib32-alsa-plugins lib32-libcanberra-pulse , so that sound would work on flash videos. I don't know if that helps , I just hope that valve moves steam videos over to html5 soon.
@Ghost99 do you have steam-meta emerged in order to pull in a full set of dependencies?
@Tele42 Yes, and I have enabled flash USE flag globally.
Problem still present in current version with STEAM_RUNTIME enabled.
In current version sound in previews is working. Maybe it was a system side problems, don't know what exactly.
Nothing extracted yet.
I can confirm problem only in Gentoo amd64 distribution.
I'm using pulseaudio sound system and sound is 100% working in my system. Adobe Flash also have no problems with sound. Even all Steam distributed games have no problems with sound. But video previews in Steam have no sound (already unmuted everything).
In stderr I have:
/usr/lib64/alsa-lib/libasound_module_conf_pulse.so does exist and readable.
Installed packages:
media-libs/alsa-lib-1.0.26-r1
media-plugins/alsa-plugins-1.0.26
media-sound/alsa-headers-1.0.25
media-sound/alsa-utils-1.0.26-r2
media-sound/pulseaudio-2.1-r1
and latest Steam