*sigh* I just found this information in the Official Bootsound thread:
"Well I did some more research on why it comes out squeaky sounding. The rate must be 22050 Hz or else it will be converted down when put on the 2x, which changes the speed of the sound. That is the cause of the squeakies!"
I know that I tried that bitrate, but I'll play with it again. I hope this is helpful.