If you move the samples that iDrum uses in a song or iDrum file, iDrum considers the file 'missing'. When this happens, iDrum will display the file's name in bold red type (the indication may differ between skins, check the skin's documentation). When a file is missing, that file's channel will not make any sound when triggered.
To relocate the file, simply use the 'replace file' button to the left of the file's name and find the missing file on your hard drive using the Open file dialog.
Related Tasks:
Locating a missing sample file.