The internet says this is the first published SID tune on the C64 using samples. Except it does not contain any samples!
The digi sounds is procedurally generated by small code blocks in the sid file, like this one:

0000 A0 05 LDY #$05
0002 A9 94 LDA #$94
0004 18 CLC
0005 69 40 ADC #$40
0007 8D F8 42 STA $42F8
000A A2 19 LDX #$19
000C B9 05 43 LDA $4305,Y
000F 38 SEC
0010 E9 01 SBC #$01
0012 D0 FB BNE $000F
0014 A5 DE LDA $DE
0016 18 CLC
0017 69 65 ADC #$65
0019 85 DE STA $DE
001B 29 0F AND #$0F
001D 8D 18 94 STA $D418
0020 CA DEX
0021 D0 E9 BNE $000C
0023 88 DEY
0024 10 E4 BPL $000A

It is basically a loop containing a a variable delay and a formula generating a new number between 0 and 15 stuffed into the volume register $d418.

There where one loop per digi-sound with different delays and formula.

No NMI interrupt used.

Screenshot of the sounds: http://imgur.com/gallery/i6pmD