Voice In Plus is licensed on a per-user basis and you can use it on multiple machines.
Case 1: If you use the same Google Account for Chrome on all your machines
You can find the Google Account being used on Chrome at the top right corner as shown above. Voice In syncs data across all your Chrome instances using the same Google Account, so Voice In Plus should be automatically enabled on all your machines.
Case 2: You are using a different Google Account on some of your machines
You need to active Voice In Plus once per unique Google account. Simply visit the Voice In Plus activation page again on your new machine. That should activate Plus on the new machine :)