I've been using soundproofing tapes since they were actually cassette tapes
I have no empirical data that they are the sole reason my dogs are unphased by sound but in 30 years of GSDs I've never had a sound fearful dog
Last night was Canada day so there was public fireworks less then a mile away and my 2 and 1 year olds laid on the deck chewing bones for 20 minutes like nothing was happening.
A few of my other tricks include active construction sites, always good because rarely will you find other dogs, road construction (especially with big equipment and jackhammers) . Both of these are great as you can control distance, start playing when you see your dog becoming alert and slowly get closer. Flirt poles work great for this. You'll also probably get a chance for some stranger contact as everyone wants to pet a puppy
Finally a kiddie pool filled with platic pop bottles is another great sound proofing tool, throw a toy in and let your pup go searching (think child's "ball room" at some of the child oriented chain restaurant"
Lastly I know why you were holding your pet at the pet store and that's totally understandable, but for the above exercises, the less handling the better. Encouraging calm voice and play is what you want, so scary things equal fun not coddling. If that makes sense.
Good for you for this effort, you'll have a wonderfully cam and confident dog with your approach