Coded using the CA65 assembler.

use like this:

call NMIDIGI_Init once for init, then call NMIDIGI_Play with a pointer (x=hi,y=lo) to a table with parameters (data start lo/hi, data end lo/hi, samplerate lo/hi). NMIDIGI_Off temporarily disables the player, NMIDIGI_On enables it again.

the various compile-time options should be self-explaining (i hope) :)

;-------------------------------------------------------------------------
; NMI Sample player (w) Groepaz/Hitmen
;-------------------------------------------------------------------------

                    .export NMIDIGI_Init
                    .export NMIDIGI_On
                    .export NMIDIGI_Off
                    .export NMIDIGI_Play

                    .export NMIDIGION
                    .export NMIDIGIOFF

withsidplayer=0
.if(withsidplayer=1)
sidplayervol=D418HLP
.endif

;0: high nibble first 1: low nibble first
firstnibble=1
;1: no nibbles
nonibbles=1

NMIDIGI_Init:
                      jsr NMIDIGI_Off

                      lda #$00
                      ldx #$00
@l:
                      sta __SID__,x
                      inx
                      bne @l

                      lda #$00
                      sta __SID__+$05   ; voice 1 ad
                      lda #$f0
                      sta __SID__+$06   ;         sr
                      lda #$01
                      sta __SID__+$04   ;         ctrl
                      lda #$00
                      sta __SID__+$0c   ; voice 2 ad
                      lda #$f0
                      sta __SID__+$0d   ;         sr
                      lda #$01
                      sta __SID__+$0b   ;         ctrl
                      lda #$00
                      sta __SID__+$13   ; voice 3 ad
                      lda #$f0
                      sta __SID__+$14   ;         sr
                      lda #$01
                      sta __SID__+$12   ;         ctrl
                      lda #$00
                      sta __SID__+$15   ; filter lo
                      lda #$10
                      sta __SID__+$16   ; filter hi
                      lda #%11110111
                      sta __SID__+$17   ; filter voices+reso

                      rts

NMIDIGI_On:
                      LDA #>NMIDIGION
                      STA $FFFB
                      LDA #<NMIDIGION
                      STA $FFFA

                      LDA #%10000001    ; enable CIA-2 timer A nmi
                      STA $DD0D
                      lda $DD0D
                      LDA #%00000001    ; timer A start
                      STA $DD0E
                      rts
NMIDIGI_Off:
                      LDA #%00000000
                      STA $DD0E         ; timer A stop
                      LDA #%01001111    ; disable all CIA-2 nmi's
                      STA $DD0D
                      lda $DD0D

                      LDA #>NMIDIGIOFF
                      STA $FFFB
                      LDA #<NMIDIGIOFF
                      STA $FFFA

                      lda #$00
                      sta NMIPOINT
                      sta NMIPOINT+1
                      sta DIGISTOPLO+1
                      sta DIGISTOPHI+1

                      rts


NMIDIGI_Play:

                      jsr NMIDIGI_Off

                      stx NMIDIGIPTR+1
                      sty NMIDIGIPTR

                      ldy #$00
                      lda (NMIDIGIPTR),y
                      sta NMIPOINT
                      iny
                      lda (NMIDIGIPTR),y
                      sta NMIPOINT+1
                      iny
                      lda (NMIDIGIPTR),y
                      sta DIGISTOPLO+1
                      iny
                      lda (NMIDIGIPTR),y
                      sta DIGISTOPHI+1
                      iny
                      lda (NMIDIGIPTR),y
                      sta $DD04
                      iny
                      lda (NMIDIGIPTR),y
                      sta $DD05


                        .if(nonibbles=0)
                      lda #$00
                      sta nib+1
                        .endif

                      jsr NMIDIGI_On

                      rts

;-------------------------------------------

NMIDIGION:
                STA NMIABUFF
                STY NMIYBUFF

                .if (withsidplayer=1)
                  lda sidplayervol
                  and #$f0
                .else
                  lda #$10
                .endif

D418NMI:        ora #$00
                sta __SID__+$18          ; volume reg

                .if (withsidplayer=1)
                  sta sidplayervol
                .endif

                .if (DEBUG=1)
                 sta $d020
                .endif

                LDA NMIPOINT+1
DIGISTOPHI:      CMP #$12                ;ENDHIGH
                BNE SK1

                LDA NMIPOINT
DIGISTOPLO:      CMP #$00                ;ENDLOW
                BNE SK1

                .if (withsidplayer=1)
                  lda #$08
                .else
                  lda #$00
                .endif

                STA D418NMI+1

                jsr NMIDIGI_Off

                LDA NMIABUFF

                RTI

SK1:

                LDY #$00

                .if(nonibbles=0)
nib:            lda #$00
                and #$01
                bne s1
                .endif

                LDA (NMIPOINT),Y

                .if(nonibbles=0)
                .if(firstnibble=0) ; high nibble first
                  lsr a
                  lsr a
                  lsr a
                  lsr a
                .else              ; low nibble first
                  AND #$0F
                .endif

                jmp s2
s1:
                LDA (NMIPOINT),Y
                .if(firstnibble=1) ; high nibble second
                  lsr a
                  lsr a
                  lsr a
                  lsr a
                .else              ; low nibble second
                  AND #$0F
                .endif
                .endif

                INC NMIPOINT
                BNE @SK
                INC NMIPOINT+1
@SK:
s2:
                STA D418NMI+1

                .if(nonibbles=0)
                inc nib+1
                .endif

                LDA $DD0D

NMIABUFF=*+1
                LDA #$00
NMIYBUFF=*+1
                LDY #$00

NMIDIGIOFF:
                RTI

(Edit: Suggestion from Algorithm)

the digiboost should be something along the lines of..

lda #$ff
sta $d406
sta $d406+7
sta $d496+14
lda #$49
sta $d404
sta $d404+7
sta $d404+14

and the nmi frequency needs to be set. eg

lda #$3c
sta $dd04
lda #$00
sta $dd05 

(Edit enthusi)

If you are in real need for cycles and have some RAM left you can replace the LSRs with:

         tay
         lda freq_table,y

and set up a table via

         ldx #0
loop2
	 ldy #16
value
	 lda #0
loop
	 sta freq_table,x
	 inx
	 dey
	 bne loop
	 cpx #$00
	 beq end
	 inc value+1
	 jmp loop2
end
         rts