Sound server
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A sound server is software that manages the use of and access to audio devices, most notably, the soundcard. It usually runs as a background process. The term could also apply to a server dedicated to audio streaming or a networked or stand-alone appliance for playing sounds and sound files.
Sound server in an operating system
In a Unix-like operating system, the main task of a sound server is performing the mix of different data streams and send out a single unified audio output device of system. This mixture is usually done by software, or hardware if there is a supported sound card.
Motivation
Sound servers appeared after limitations in Open Sound System were realized. OSS is a basic sound interface that due to limitations caused problems to developers and users, such as multiple sound sources playing simultaneously.
Prior to sound servers, advanced sound features such as network sound, multiple sound cards, and a centralized control mechanism were not available.
Sound servers add these features by running as a daemon system working in the background. They receive calls from different programs and sound flows, mix the streams, and send out to the audio device raw audio. Users can also use these calls to configure global and per-application sound preferences. These servers take advantage of features provided by generic sound card interfaces, such as ALSA.
Diversification and problems
Currently, there are multiple sound servers, some are focused on providing very low latency, and others, offer general features for desktop systems. This variety has resulted in some arguing that a standard API is necessary to focus efforts on a single server sound. With the diversification of servers, a user can choose which option to use. Developers must attempt to accommodate these options by writing code compatible with the various sound servers available.