Summing stereo and mono channels passively
Your mono signal should mix with left stereo and right stereo using resistors for simplicity: -

L music \$\rightarrow\boxed{\text{  1 kΩ  }}\rightarrow\$[L output]\$\leftarrow\boxed{\text{  1 to 10 kΩ  }}\leftarrow\$ Mono signal
R music \$\rightarrow\boxed{\text{  1 kΩ  }}\rightarrow\$[R output]\$\leftarrow\boxed{\text{  1 to 10 kΩ  }}\leftarrow\$ Mono signal

Try using 1 kohm resistors but be prepared to reduce the mono signal resistors to 10 kohm.