From a post on page 15 of the Murphys Models 121 Class thread:
MM0567 Class 121 DCC sound chip SRP €125.00 EMD 8-567CR NT (Sound Decoder representing the Sound for these Locomotives when delivered with their *GM 567 CA 8-Cylinder, twin bank, blower scavenged, developing 950 hp gross at 835 rpm, 875 hp traction. - Prior to engine transplants)
MM0645 Class 121 DCC sound chip SRP €125.00 EMD 8-645E NT (Sound Decoder representing the Sound of these Locomotives from Transplanted Engines. Engines from from withdrawn C Class (201) Locomotives were installed in Class 121 Class Locomotives as and when it became necessary to replace engine power packs. *GM B-645 E 8 cylinder. twin bank, blower scavenged, developing 1,100 hp gross at 900 rpm, 1,040 hp traction.
*Information taken from Irish Locomotives and Rolling Stock of Coras Iompair Eireann and Northern Ireland Railways - O.Doyle ans S. Hirsch
I'd reckon the 567 sound chip would be applicable to the older livery you have ordered, but I stand to be corrected on that