Is it just in one channel (left, right) or both? If it's just one, then you have a bad splitter, but if it's both, it's more likely that the splitters are causing issues with the way the PS1 and PS2 handle audio outputs. If I remember right, the PS2 has a sort of idle power state (you see a red light lit up) when the system is off - you're probably losing impedance (I think) on the PS1 signal since the PS2 is maintaining "active" power and still influencing the overall scheme somehow. Could be wrong on that, but it seems most likely to me.
You'll need a bigger switcher, or perhaps a pair of switchers in conjunction to cover all of your system. Y-splitters are generally a poor way to handle things as they generally result in at least a little signal loss.