Could have a couple of small issues there.
The iPod touch is just a few millimetres shorter than the iPhone, so assuming you aligned it to the home button at the bottom you would be left with a (very) little unprotected space at the bottom and top. And as also mentioned there would be no speaker hole on the iPod touch screen protector. I wouldn't have a clue as to whether or not that makes a whole lot of difference to the sound.