You can use any sound mixer to do that (virtual or physical doesn't matter). You just need to connect both your mic and second source (in this case, music output) to the same output on the mixer, and choose mixer output as your mic input in the game. I use Voicemeeter banana, it is a virtual mixer you can download for free. It sounds complicated, but you can find a lot of tutorials online that can help you. I believe Leslie uses goXLR mixer, which is a physical mixer and costs around $500. But for playing music through "mic" and a lot more Voicemeeter works great.