Hi there Hanomag.
1) I have both .mp3 and .ogg in my Gramophone folder so I guess both of them works. Maybe the built in mp3-player doesn't like the mp3-files you tried with for some reason. You might try with mp3's or ogg's from another source to sort that out.
2) Your harddrive space is the only limit as far as I know!
3) As for the volume it's correct that 0db is loudest and -40 is the opposite.