There are a few means of distorting your voice for anonymity, but a sophisticated enough algorithm can reverse anything you try. The most secure thing you can do is to blend things, pitch your voice, add audio distortion, use multiple samples, shift accents / cadences / ways of speaking, change modulation at random points in the audio, etc. If you’re trying to evade the government they will eventually figure it all out, but your average schmuck will probably never get a fully clean audio out.