Finder/spotlight is missing some files

I have a bunch of songs in lossless uncompressed audio format .aiff. I would like to convert them to lossless compressed audio format .m4a (ALAC). That's easy enough, especially because I have all the .aiff files stored in one folder (organized in folders by album).

But Finder/spotlight is missing .aiff files when I type ".aiff" in the search box. If I search for "aiff" in the extension field, even less files show up. Clearly, it's not finding all my files, which is important because after I convert, I'm going to delete all my old .aiff files. If Finder can't reliably find all my .aiff files, I might delete .aiff files I haven't converted yet. It's also not reasonable to go through each album folder manually (unless that's the only option).

Is there a more reliable way to grab/find all files of a certain extension in a folder?

 

The folder containing all these files are located in my home directory.

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Also my only issue now with spotlight is that even if I do re-index, how do I tell if the count I get after is correct? I wish there was another way (maybe in terminal), so I can compare.