I would first create a new scene and give it a name such as "College" then in the new scene click the + to add a new source.. and choose "Window Capture" from the list.. >> "add new" at the next dialogue>>> select the relevant window and click OK
To get the sound click the + to add a new source and this time choose "Audio output capture" and then select whatever device it is that your sound comes out in the list.
Check you have levels in the mixer and the right screen displaying in OBS and then hit record
If you want to set this up in advance you could choose any window to capture, then when the lecture goes live , right mouse on the words "Window Capture" in the sources menu and choose properties and you can then change to the right window to record.
hope this helps