Great, thanks for checking.
Please open the script in Script Editor and scroll down until you find a section that starts -- calculate file extension followed by a number of else if statements and locate these two lines:
else if k = "MPEG audio file" then
set ext to ".mp3"
Replace them with:
else if k = "MPEG audio file" then
set ext to ".m4a" -- custom tweak for mismatched file types
-- set ext to ".mp3"
That should manage to link the old references to the current files.
Double-checking the details you posted also suggests that your actual file doesn't have the leading track number that would normally be expected. Find this section of the script:
-- evaluate file name
set theFile to ""
if track number of aTrack > 0 then
set theFile to theFile & track number of aTrack & " "
if track number of aTrack < 10 then set theFile to "0" & theFile
-- add leading disc number if implied
if disc number of aTrack > 1 or disc count of aTrack > 1 then set theFile to (disc number of aTrack as text) & "-" & theFile
-- or always add leading disc number
-- if disc number of aTrack > 0 then set theFile to (disc number of aTrack as text) & "-" & theFile
end if
and add this line after the final end if :
set theFile to ""
which is an alternative to either cutting out that block or commenting out each line. The block builds the start of a file path with the track number, a leading zero if needed, and then a disc number if implied. Adding the line resets to an empty string, which is then going to get the name of the song added on to it, the file extension, and then the path the folder that should hold the album. Obviously ignore all this if that was a typo and the real path is actually:
/Users/davidschepard/Music/Music 1/Media.localized/Music/The Decemberists/Castaways And Cutouts/05 Odalisque.m4a
You should now be able to select multiple missing files and run the script to repair the links of each automatically.
As to what may have happened I can see that it is perhaps possible that you could have removed local .mp3 files in favour of streaming from the cloud at some point, then downloaded AAC/.m4a versions from Apple Music, and then later restored an older backup of your library database that still refereed to the old .mp3 files. Perhaps you can recall how it actually went down but regardless I hope the script helps you repair what you have now.
tt2