Just having a go at it myself:
Calamatous rudder failure is fortunately very rare, say 1 in 500 aircraft, over eachs' say 20,000 hour life. That's a 1 in 10 million chance per hour, or a 1 in 100 million chance in any few minutes.
A CVR failure is more likely, say once in a thousand hours per aircraft, or a 1 in 10,000 chance in any few minutes.
Assuming they are unrelated, the chances of a CVR failure being followed within minutes by a mechanical rudder malfunction, is the one odds multiplied by the other. I reckon that gives odds against mechanical failure of 1 in 1,000,000,000,000 or one in a billion.
I may have made wrong assumptions, or got the zeros wrong, but the point is that the chances of the MI 185 tradgedy being due to a mechanical rudder failure, immediately preceded by CVR failure, is remote.