$4015 is the voice enable register and uses bits 0 to 4 to enable Pulse 1, Pulse 2, Triangle, Noise and DCM respectively.
Binary Hex Voice
%00000001 $01 Pulse 1
%00000010 $02 Pulse 2
%00000100 $04 Triangle
%00001000 $08 Noise
%00010000 $10 DCM
To figure out the value to write to $4015 just add up the values. For example, to enable the two Pulse voices, you write $03 to $4015 ($01 + $02). The APU is generally initialised with a value of $0F which enables the first four voices ($01 + $02 +$04 +$08 = $0F).
DCM differs slightly in that it requires you to turn the voice off and back on again to actually play the sample. In Don's code, he's not using any of the other voices so he doesn't need to turn them on, hence the $00 write (and probably $10 somewhere further down the code to start the sample playing).
So for you, instead of writing #$00 to $4015, write #$0F. This leaves the first four voices on but clears the enable bit for DCM, as is required. Then to start the sample write #$1F to $4015.
Make sense?