An input device such as a microphone is a type of audio source, while an output device like a speaker is the audio sink.
What is a sink in Linux?
A sink is a filter-like program that consumes standard input but emits nothing to standard output. Again, its actions on the input data are controlled only by startup conditions.
What is audio sink in GNU Radio?
Allows a signal to be played through your speakers or other audio device.
What is a video sink?
A media sink is the destination for one or more media streams. Media sinks fall into two general categories: A renderer is a media sink that presents data for playback. The enhanced video renderer (EVR) displays video frames, and the audio renderer plays audio streams through the sound card or other audio device.