Okay, I've been searching for a while now and I'm only mildly annoyed by this.

I have a mixture of .flac and .mp3 files due to my downloading in mp3 format and taking my boyfriends things, who likes his stuff in flac. I'm saying this so if there might be a problem with perhaps our file sharing or the way we give the files to each other?

Normally we just open Rythymbox and I take what songs/albums I want and most of the songs work. I can play them just find. I recently did an overhaul to fix the tags and thus fix my replay levels. The albums refuse to be put into Music Brainz, MP3Tag, and when I put them in the Terminal to run a replay gain on them I get this.

nikki@nikki-laptop:~/Music/Akitaka Tooyama/Complete Sounds of Katamari (disc 1)$ metaflac --add-replay-gain *.flac
*.flac: ERROR: reading metadata, status = "FLAC__METADATA_CHAIN_STATUS_ERROR_OPENING_FIL E"

The FLAC file could not be opened. Most likely the file does not exist
or is not readable.
nikki@nikki-laptop:~/Music/Akitaka Tooyama/Complete Sounds of Katamari (disc 1)$

I can't do anything to read or 'find' the files but, this is the part that baffles me the most, it plays just fine in my music player. It doesn't do anything weird, a little loud and some weird tags are in it. I know some about this stuff, but not enough, please use basic language.

Did something possibly happen during the transfer process? Is there some way to fix it or keep it from happening again? (This is half my music library, so very annoying for me to have to keep switching music levels up and down constantly) Would anyone know why it would do this but play fine? I'm baffled.