It's not unheard of for wifi and bluetooth to have compatibility issues. I've had some myself, and read a number of posts from other folks who've had trouble when running both simultaneously. I know a lot has been done in the last number of years to make them co-exist better, but it still is a possible source of problems. Other sources of interference can be other BT devices or microwave ovens.
As I understand the problem, when a Bluetooth device encounters interference on a channel (because wifi is on the same frequency), it deals with the problem by hopping to the next channel and trying again. This can cause a slowing or stuttering of the transmission. I'm wondering if this is what's producing a "garbled" sound for the OP. Best way to test is to turn off wifi and see if the problem goes away.
I'm also wondering if the OP's change to the Razr also was a change to ICS. If so, this might be an ICS issue which Plantronics might address.