I just installed Rock Band: The Beatles, and just have the SingStar mics.
I also have an Eyetoy connected.
Even if I disable the Eyetoy in the PS3 audio settings, I get really bad feedback when I use Rock Band.
The only remedy I have found is to disconnect the Eyetoy when I play Rock Band.
When it is connected, even though I have disabled the mics from it in the PS3 settings menu, two mics still appear for it on the game screen.
Any help/advice would be appreciated.