Separate human voice from Music

Hi, I have two audio files. One is human voice and other is music file. I am tasked with:

1. Mix the two files
2. Design fillter to extract audio from it.

I mixed them by:

y3 = (y1 + y2)/2; i.e. taking average of two audios. Now as for voice extraction I am limited to use fir2() and filter() functions only.

The audio voice extraction method is based on two steps:

1. designing a lowpass filter to separate frequency components below 4kHz
2. designing a notch filter to get rid of high amplitude frequencies of the music below 4kHz

I tried multiple approaches but can't extract audio file from music. Kindly help me out on this.

These are the audio files.