Well, after some search on the Internet, I've found some information on the signatures to be used with multimedia files. Caution, all those strings are case sensitive!! All strings are 4 characters long and don't include the single quotes I used to make things clear.
For an undetermined target computer (that is, for a CD-ROM you publish to undetermined users), one should use QuickTime, that is 'TVOD'. On the other hand, for sound files, if you are sure that the target computer has SoundApp, you could use 'SCPL'. If you know that Real Player is on the target computer, you can use 'Pnst'. Caution, RealPlayer is known for changing the file type of MP3 files from whatever it is (see below) to 'PNRA', to prevent other programs to open the MP3 files afterwards.
Wave files (PC extension WAV). Macintosh signature 'WAVE' or '.WAV'.
MP3 files (PC extension MP3, but there are also others). Macintosh signature 'MPEG', 'MP3 ', 'Mp3 ', 'MPG3'. Also seen 'PLAY' and 'SwaT' for MP3 files.
QT Movies (PC extension MOV). Macintosh signature 'MooV'.
One could therefore add to the signature file used at this time following lines (to cut and paste in the *.sig file).
MP3,MPEGTVOD,MP3 sound file
WAV,WAVETVOD,Wave sound file