you can't proof if voice recordings are changed unleast you know very much about recording, audio and digital signal processing and even the you can't tell 100% that there has nothing been done with it.
if you don't trust the source from where/who you have gotten the recording you also shouldn't trust the recording.
thats why an audio-sample is just used as supporting evidence never as the main-evidence.
there is one thing on all digital media one can do to recognize that they may have changed this thing is called checksum or also digital signature.
it can bee used to proof that a digital media didn't change from when the author created it till an other one recives it but it can't proof that the author himselfs didn't change it.
i think the best answer will be it doesn't belong to you to proof wether the recording is orignal or not.
it is the same as if it would be a audio-tape the one who have to decide wether it is original or not are the forensic-analysts of the police and at last the jury and the judge.
as long as they all accept it as a evidence it doesn't matter if it really is a original or not.