PulseAudio

PulseAudio is a sound server system capable of working across networks. It is used for audio multiplexing, or allowing different programs to access the sound driver at once.

In other words, PulseAudio acts as a middleware between applications and the kernel sound system, ALSA. It also connects sound sources (inputs) to sinks (outputs).

PulseAudio was created by the creator of systemd, Lennart Poettering.