Add one (or more) Audio sources to OBS
Most video streaming/recording programs nowadays only allow you to select the Microphone device which you want to use for your recordings/streamings. Luckily many programs and even some games allow us to select which output device they will use, so we can exclude audio off our recorded material. OBS even goes further, it not just allows us to select which “desktop” audio device we want to use and which microphone, but it also allows us to add several more audio sources(recording devices, webcam/capture card audio, etc.) using the DirectShow Audio Plugin. This can be useful if you want to listen to music through your speakers and hear voice-coms and game-sound in your headset, or if you want to add music using an external device, or if you just want to sometimes play a sound-effect or similar on stream. In combination with a software like Virtual Audio Cable, as an example for virtual sound devices, you can add probably unlimited amounts of audio sources actually, but someone would have to test this. After you installed the plugin, you can add a “DirectShow Audio Source” which should give you a similar configuration like this:
You will have to test a bit, to my experience you can select all recording devices of your PC: Line-IN, Microphone, etc. as well as the mentioned Virtual Audio devices or Webcams, Capture Cards. I had problems using playback devices and it sometimes depends on your Webcam or Capture Card if the Video part has to be initialized for the audio to work. So for example if you want to play music using your smartphone or mp3-player, connect it to the Line-IN port of your PC and select the Line-IN in the dropdown. Either Output audio to stream or to your desktop(so you hear it as well) and tweak the volume as needed. You have to add it to every scene you want to “hear” it in. Another “specialty” of the sources that get added using this plugin, they are independent of your desktop and auxiliary volume sliders. You can mute your desktop sound an microphone and still play music or whatever on your stream using a DirectShow Audio Source. As mentioned, you can add more than one of those sources at once and you can also use them as global sources of course.
In the Multiplatform version of OBS you can right away select more than the usual 2 devices. The option to add specific devices to each scene is also already included. So if you run into problems with OBS and the DShow Audio Plugin, you might want to give OBS-MP a try.