This looks like a modern stake center and this is on the pulpit control correct?
This is the control for what inputs to use as well as a level control for those inputs. You are correct in that the English/Spanish function is for the satellite language as well as a level control (independent of the main). Your "MEDIA SOURCE" inputs may be on or at the pulpit, or they could be back in your tech RAC. For example, Our RAC has media inputs that we use for early prelude or post prelude music when our Piano/Organist is late or finishes a little early. The leadership can select the background music as well as the level. I use an old phone with 1 hour of piano arrangements playing in a loop. Ours has a duplicate control in the RAC. The input plugs are either 1/8" jacks or RCA. This is great if you want to feed reverent music though the building for funerals, buildings cleaning etc.
Also in these buildings you will find similar controls in the RS room and/or Primary room with a similar MEDIA SOURCE input of a 1/8" jack near this control for independent room audio. We use this to play music locally, or use local MICs. For example RS meal or visiting authority conference meal. We use it extensively in our Stake center. I can share all kinds of ways you can use these inputs! We also play music though them in the cultural hall for talent nights as well as a little more energetic music for basketball and volleyball. It is VERY useful.
Here is a reference:
https://tech.churchofjesuschrist.org/wi ... ther_Rooms ...except I think your picture is for the chapel or the entire building "Room"