Inside the !DCDRes directory you will find a file called Setup and in this file a section [FileTypes]. On startup the application scans this section for lines in the form:
filetype = loader1, loader2, ...
where filetype is a RISC OS filetype, loader1 the first player that will attempt to play the file, loader2 the second one if the first refuses, etc.
For example CB6 = tim, qtm tells DigitalCD to accept filetype CB6 and play such files with the TimPlayer module, or with QTM if TimPlayer refuses.
You may add, modify or even suppress filetype mappings at will as you can alway find back the default version of this file in the directory !DigitalCD.Copy.