Dying Day-confused plot (contains spoilers)

Skipling's ex-wife, and her co-conspirators want her rich second husband out of the way, so they can enjoy his wealth-so they fake his distinctive accent on a tape as a party to a supposed death plot against Skipling, so he can be implicated as a murder conspirator and jailed. So why do they then substitute the tape without the faked voice, or even discussion of the murder plot, before Skipling takes the tape to the police?