If you have a Clip+, and then install the Rockbox firmware, there is an audible difference in the sound because Rockbox fixes the incorrect playback speed of the Sansa firmware. So yes, firmware can indeed change the sound, even in the mainstream devices.
Is that documented anywhere? Daft Punk's "One More Time" sounds the same to me on the stock operating system compared to Rockbox, no EQs applied.
Of course this is time dependent as it takes a while to switch from one OS to another, but from memory they sound identical to me with approximate volume matching.
I don't know if it makes a difference, but I'm using the Clip Zip instead of the Clip+.
Edited by miceblue - 6/30/13 at 3:09pm