On my desktop running Oneiric, sometimes there's no sound at all, but it would work normally at other times. My setup is built ground-up from minimal Ubuntu, but since sounds work fine, at times, I don't think it is a backend dependency issue. When it works, it will play anything from regular audio files and movies to YouTube Flash players.

For the record, I installed LXDE with the alsa-base and alsa-utils packages which are the only audio-related dependencies for the lubuntu-desktop.

For a while, I also used persistent Oneiric live USB, and do not recall any sound issues.

It's one thing to not play sound entirely, but playing sound only under some very unclear circumstances is something else. Please help me diagnose it.