There are few different ways to do this, both software and hardware based ones. Before windows 10 it was more common to have "What U Hear" or Stereo mix enabled by default in windows if the drivers supported it. This meant that a sound output could also be used as an input easily directly in windows. In windows 10 for the devices that have this feature supported by its driver it is now disabled by default and have to be turned on but it seems that support for this feature is much less common nowadays.
The second option is
Virtual Audio Cable and although the website looks very sketchy it works, but it may require some troubleshooting if you run into issues.
The third option is to let the broadcasting software OBS take care of the sound mixing for you. This is only possible on MFC if you are using the OBS HD streaming beta which to be honest isn't a 100% perfect at the moment so you might want to stay away from this one until it functions properly.
The fourth option is to get an actual audio cable and run from the speaker/headphone jack on your computer and plug it into line in or mic depending on what you want to use as a sound input, then people will be able to hear what you should have heard. To be able hear sounds yourself with this option you will need to use a slitter and plug that into your headphone/speaker jack first and then plug speakers/ headphones into that and the audio cable that goes into the microphone/line in.
The fifth option is using a mixer board. I use a USB mixer board and I normally use a dac and amp to get sound to my headphones, the dac and amp works very much like a sound card, but it only has sound output. So I run an audio cable from the dac and amp to one of the line ins on my mixer board and because it is USB it shows up as a sound input device in windows. So If I want to play music I just turn down the volume for my mic on the mixer board while leaving the volume on line 3/4 where my dac and amp is plugged in turned up at a comfortable level. This is probably the best option because it gives the most control over the sound that you broadcast, but it is also the most expensive one, because you need at least a mixer board, XLR microphone, headphone or speakers and an audio cable. This would probably cost around $200 dollar for ok quality gear. If you are starting out there are probably other things you should invest in before you try this and to be honest I don't think I have seen any MFC model do it this way. It might look a bit complicated but it is actually quiet easy to get it to work.
Options 1, 2 and 4 are probably the ones that you might want to look into if you are on Windows, if you are on a Mac I have no clue what the best option is.
