Well, I've researched all of the gramophone related threads here and can find no definitive list of attributes that a file has to have in order to work with SH4. Apparently .ogg, .mp3, and .wav will work. The only things left are the more detailed prperties like the bit resolution, the sample rate, and the bit rate.
Wav files ripped directly from CDs should have a 16 bit resolution, sample rate of 44.1 KHz (often simply listed as 44), and a bit rate around 1411.
Mp3 files should have a resolution of 16 bit, a sample rate of 44.1 KHz (also listed as just 44), and a bit rate of at least 128kbps . . . that's to be on the safe side; lower bit rates may work but I can't say for sure.
Also, here is a link for a huge download of radio mods. You can just use the music files from that mod and stick them in your gramophone folder if you'd rather not mess with the radio part. At least these files are already formatted to work with SH4. With some searching you will find more links to WW2 era music that users here have made available.
http://www.subsim.com/radioroom/show...ght=gramophone
Here is a thread that discusses the file format and properties of music for the gramophone. There is even a link in there to a freeware sound editor that you might want to experiment with in order to change the properties of your non-playing files.
http://www.subsim.com/radioroom/show...ght=gramophone