If any specifically-mentioned assets are disposed of (sold, given away or lost) by the decedent while the decedent was still alive, those assets would no longer be considered part of the estate, regardless of what the Will might say.
Any specific gifts for these assets would simply lapse and no “payment” for such lapsed gifts would be made to that named beneficiary.
The Executor must not make any ‘substitutions’ when disbursing assets of a Will and estate: just follow the stated directives and if an asset cannot be found, skip that listing.