Volume Controls
Sounds like you are excedding the power spec of your volume controls and burning them up. To prevent this issue, check for a potentiometer which sets the maximum volume and power to each set of speakers. Since you have a multisource / multizone system. You probably have Cat5 (and possible speaker wire) from your distribution amp to each volume control. You are suck with the same manufactuerer and line of VC to make your multizone feature work, but they might offer a higher power version of the volume control. This stuff is usually expensive, so you might check if the damaged VCs are under warranty.