The US code is very dry when reading, but can be very illuminating. http://www.atf.gov/regulations-rulings/laws/
I stumbled upon it a while back when looking up something else. A person needs to be a nerd's nerd to read this crap.
Here is the part about refunds.
TITLE 27 CFR CHAPTER II
PART 478—COMMERCE IN FIREARMS AND AMMUNITION
479.172 Refunds.…..When an applicant to make or transfer a firearm wishes a refund of the tax paid on an approved application where the firearm was not made pursuant to an approved Form 1 (Firearms) or transfer of the firearm did not take place pursuant to an approved Form 4 (Firearms), the applicant shall file a claim for refund of the tax on ATF Form 2635 (5620.8) with the Director. The claim shall be accompanied by the approved application bearing the stamp and an explanation why the tax liability was not incurred. Such claim shall be filed within 3 years next after payment of the tax.
Refunds are not automatic and when buying a silencer from a dealer, it is a good idea to agree on what happens if the transfer is not able to be completed just as when buying anything that can cost hundreds or thousands of dollars. I have only used the ATF form 1 (to make a firearm) when applying for a tax stamp, never the form 4. The process is a little bit different than the form 4. But in my opinion as long as you can pass the NICS check and live in one of the 37 states that allow them, the approval process is practially a rubber stamp.