Let me guess:
- Your music source is its own separate thing in OBS, and you send it to the Monitor so you can hear it.
- Your game is outside of OBS, so you have the Desktop source pick it up from the sound card's output.
- You send OBS's Monitor to that same sound card because that's what you're listening to.
That's a feedback loop.
The Desktop source grabs the signal VERY late in the process, just as it's leaving the computer entirely. After mixing everything together, and after any speaker effects. Because it's after the mixing step, the Desktop source also includes OBS's Monitor, and anything that you've sent there, like the Music in your example.
To fix that, you need to break the loop. Doesn't matter how or where, just as long as you do. The best option that I can think of at the moment is to not send anything to the Monitor at all, which might mean that you can't hear the music as you currently do it. But you can use an external player for that, and pick it up with the Desktop source along with the game.
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In most cases, it helps to draw a diagram of how the signals flow through the entire system. Boxes with arrows. I've attached one that I made for one of my rigs. It's a lot more complicated than what you're doing - I have two simultaneous instances of OBS that each do different things, and I'm doing all of my audio work externally to both of them - but it's the general style that counts. Boxes with arrows.
The grid in the middle, with different squares highlighted, shows different connections for different scenes. Everything else stays the same. The grid's columns are its inputs, and its rows are outputs. For everything else, audio (black arrows) always enters on the left and exits on the right, and control signals (red arrows) enter on the top or bottom. Control signals can be created as audio, but its their use by the destination that makes them control instead.
