Sometimes the voice from Geralt is different, I think it is from the alpha version.
Im playing in english.

Sorry I don't know about the alpha version... but even then I can't say I feel Geralt has two different voices in game.

However, I do find that there are volume level problems when Geralt talks to others. It often occurs that in the same scene (e.g. camera/framing doesn't change) Geralt or his dialog partner lowers the voice such that you can barely hear him/her only to yell come the next line. First I thought the voice actors where using it for dramatic effect but it appears randomly at points in the conversation where it doesn't make sense.

Not a biggie though... turning down the background music fixes it for me.

I shall keep an open ear however, if I can confirm your findings.