Is masturbation, upon hearing one's traveling wife speak ordinary words through a voice recording, considered a violation of Islamic law? And what is the evidence for that?

Masturbation by hand or similar means is absolutely forbidden. However, the emission of semen merely by deriving pleasure from hearing one's wife's voice, looking at her, or by her hand, and so forth, carries no blame. This falls under the general category of permissible enjoyment, as Allah Almighty says: {Except from their wives or those their right hands possess, for indeed, they are not to be blamed - But whoever seeks beyond that, then those are the transgressors.}