That’s Tom’s voice.
I believe 声がする actually means “a voice is heard” . “That’s Tom’s voice”, isn’t that それはトムの声です ?
Disclaimer, I have been failing to learn Japanese for ages.
がする suggests that something is present and typically coming from something else. (eg. においがする、 あじがする、 かんじがする) so in this case Tom’s voice is present and the speaker can hear it.
According to my dictionary, 声がする means either “to speak” or “to hear a voice”. So this could be translated as “Tom speaks” or “I hear Tom’s voice.”