I don't know what's causing this but last night my notification and ringtone sounds are playing songs that I have in random. Anyone else having issues like mine?
The only time my ringtone is different from my usual one is when the SDCARD is being mounted on the computer. The mp3s on the sdcard can't be accessed in the phone, so it just plays another ringtone instead.
But it isn't random like you describe, so maybe you have some other problem..