The SPU2 is the Sound Processing Unit for the Playstation 2, and works
a lot like the sound card in your own PC; albeit still quite unique in
its approach to mixing sounds/voices and the programmable interface it
provides for that. But the SPU2 is more than just sound. It's one of the
more reliable timing mechanisms on the PS2 and games tend to use it as
such. Without at least basic SPU2 emulation, no games will boot at all.
This isn't too surprising if you understand how console hardware
typically works, but what might be surprising is realizing how many
games won't boot even with what appears to be fairly competent SPU2
emulation.