Yeah, there are also other small inconveniences with adding/exporting/importing custom sounds and sounds packs.
Anyways, I'm not sure if you did found the corrupted file or not (not clear from your post), but in case you didn't, you could use the
MsgPlus Sound Library Fixer for that.
You can find this out in two ways: either you first remove all the custom sound data from the registry (do not use Plus! for that, otherwise the files will also be deleted) and then use the Sound Library Fixer to re-add them all.
Or you simply scan your sound library with the Sound Library Fixer and compare the given list with what you'll see in the Sound Library in Messenger Plus!.
This because the Sound Library Fixer will not read in, nor add, corrupted or invalid sounds.