The problem here is that the best way is not a hardware mixer. The best solution is a software mixer. All you need to do is put your music in a DAW, and listen back through the DAW while recording your voiceover. The DAW acts as the mixer. I see no need here for a hardware mixer.
If you don't have a DAW, you can download Audacity for free. I've used this with some success using Blue's Yeti USB microphone.
The OP was not being criticized, we were trying to help him with a solution that actually works with existing equipment. If a software mixer won't help, or you don't 'like' DAWs, then I'm sorry, and wish you luck in finding a solution to your problem.