Print out the attached STL. It enables you to attach CD steppers in proper orientation.

Now, mirrors are from old scanner. This is somewhat important as normal mirrors can be used but they have mirror surface behind the glass and laser beam refracts and as mirror moves it smudges the beam.

Scanner mirror has mirror surface in front of glass and has no such issue.

Cut mirrors to size.

Before hot gluing the mirrors power everything up with 5V power supply (excluding the laser) and connect the amplifier to sound source. Place a piece of sticky tape as flags on the CD steppers instead of mirrors to see in what orientation they are and try to make them at 45 deg and 90 deg to each other. Reference picture is attached.

Once satisfied hot glue the mirrors and cautiously power up the laser to check your work. Use laser safety glasses while doing this.

And that is it. Use line out from computer as input to amplifier.

With music is playing turn on the LM317 board, laser and together with amplifier slowly increase the power until you see laser dot moving.