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The Advanced Linux Sound Architecture, shortened to ALSA, is the default sound system for most, if not all GNU/Linux distributions. ALSA replaced OSS after a new version of OSS was released under a non-free licence.
Usually on minimal GNU/Linux installations, the module dmix is sufficient to allow multiple programs to play sound at the same time. Most ready-made GNU/Linux distributions include PulseAudio and enable it by default to allow network transparency and per-application volume control, although many users exclude it because it is buggy and not worth the hassle. PulseAudio is now required to use Skype on GNU/Linux.