On the face of it, pretty serious. When you really think about it -- still pretty serious. However, if a 'hacked' voice message has the potential to change government, then logically it must contain something which the incumbent government does not want publicly known. Ergo -- the public should know, and a service has been performed.
In that particular (and grossly oversimplified) scenario though, the (replacement) incoming government would seek a much more cosy relationship with the media in order to avoid falling into the same trap. It would not naturally occur to them to think 'better not do dodgy stuff, might get rumbled' -- they will always think 'might get rumbled, make sure nothing happens if we do'.
That is the nature of the political class with which we have lumbered ourselves.