Hi Adam,

Here is one way. Create a function to perform the operation on one file

audio[file_] := 
 Export[file <> ".wav", #] &@ Audio[InverseSpectrogram[Import[file]], SampleRate -> 8000]

Test it to make sure it works as expected. Note that the output will be written to whatever Directory[] evaluates to with the same name as the input file with .wav appended. If you want a different audio format change the file extension in the audio function.

Assuming all of the input files are in a single directory, and that they are .png

SetDirectory["/path/to/directory"];
files = FileNames["*.png"]

audio /@ files

If you have millions of files it is going to take a while. Use ParallelMap to speed it up.