Can email notification sounds be repeated on an HTC Thunderbolt until the message is opened?
To the best of my knowledge, no but...
I have created a solution to this problem but it only works while you have a computer, any old computer should work, on the net.
It works by creating calendar appointments for emails. You can do it for any email but that results in way too many notifications for me. I use a collection of Gmail filters.
By creating a Gcal appointment, you get to use programs aimed at calendar notifications. They tend to be much richer than email notification apps. You can search on Google Play for such apps. I can only vouch for Calendar Snooze, the one that I use.
You can read more about Gmail2Gcal or simply test the app my sending an email to firstname.lastname@example.org. The demo will send you an email with additional information.
Gmail2Gcal is open source and free for your use. You can use issue page, found on Gmail2Gcal's main page if you have problems. I'll try to support it as best I can.