Being as how this isn't Rockbox related at all - this will be your one and only answer before I lock the thread.
There are no replacement firmwares for the iPod Touch.
You *can* however (provided you get a 1st generation Touch, and not the 2nd generation), jailbreak it, and then install either dTunes or PwnPlayer - either of which will play music from the filesystem as well as the iTunes database on the device.