Can I retrieve files from the voice recorder app after they've been deleted? Is there any way I can retrieve files that were accidentally deleted from the voice recorder on an HTC Thunderbolt?
If the files were stored on a physical microSD card, you may be able to use file recovery software on your computer.  There are a number of undelete and recovery software options, but many cost money. http://pcinspector.de has a flash memory recovery tool for free (Smart Recovery handles flash memory).

Writing more data to the card increases the chance that the file will be overwritten before you recover it.