I've seen one romhack that somehow hacked different sound samples into smw, but only one, and I have no idea how they did it. I wish I knew how, because I don't like smw's default instruments at all.
Sample editing is pretty trivial nowadays. There is a tool that can do it on SMWC called "SMWS". However, you need to manually set the key, the pitch, as well as picking a DEFAULT sample to replace (and let us remember that default samples are what make up the sound effects of the game). In other words, you'll need to be very careful of what you replace.
Oh. Did I mention that the size allowed is very small?
Either way, if you can use Audacity or another editing program to extract the samples of raocow's voice, as mentioned earlier, and cut it down small enough to make it into a loopable WAV, you can probably insert them. I wouldn't mind doing it, actually, if no one else wants to.
(on a semi-related note, I actually made an SPC that used cricket sound samples a long time ago. You can hear it
here.)