Try;
;@EXEC_ONCE "/drive0/var/pcmplay < /drive0/var/systemcheck.pcm"
(ie add the double quotation marks)

If that doesn't work, try creating a bash script, eg /usr/local/bin/startsound;

!#/bin/bash
/drive0/var/pcmplay < /drive0/var/systemcheck.pcm
exit


and then using

;@EXEC_ONCE /bin/bash -c "/usr/local/bin/playsound &"


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Mk2a 60GB Blue. Serial 030102962 sig.mp3: File Format not Valid.