Sound Server

A Sound Server mixes different input signals together and sends it to a single audio output device. This means that not only one audio source can be played but multiple (usually from multiple applications). This might seem a bit weird at first when comming from something like MacOS or Windows, however those systems do this aswell, just that you don't have a choice about what audio server to choose.

+-----------+ +---------+
| minecraft | | discord |
+-----------+ +---------+
       |          |
       |          |
       V          |
+--------------+  |
| audio server |<-+
+--------------+
       |
       |
       V
+--------------+
| linux kernel |
+--------------+
       |
       |
       V
+------------+
| sound card |
+------------+

The audio server sends the data to the kernel of the operating system and the kernel then sends it to a sound card. Note, that some applications don't need an audio server and instead talk to the linux kernel directly (ALSA). This is described in here.