Opus has superceded CELT but is not compatible with CELT files, which were always experimental. The bitstream got as far as a soft freeze, but early .celt files might be incompatible with later decoders.I'd suggest the easiest way*, if you have a Windows PC, might be to obtain foobar2000 media player/converter and download and install the foo_input_celt plugin.If it plays the .celt files properly, then you can right click the file(s) and Convert... to use any encoder you've set up.Search these forums or knowledgebase (Wiki) for how to set up LAME as an MP3 encoder for foobar2000 (also known as fb2k). A VBR quality setting from -V5 (~130kbps) to -V2 (~190kbps) should provide pretty good sound quality, and foobar2000 will warn you that transcoding from lossy to lossy will not increase quality, and may make it audibly worse, but will still let you do it.(* The celt-codec.org site only contains source code as far as I'm aware, so tracking down a commandline decoder binary might be tricky - it's not on rarewares.org, though I didn't check reallyrarewares)
Would this be a copyright violation?
It might be possible to compare it to a CELT file made by one version of the CELT encoder using a hex editor. Very quickly I took a look at it in Notepad, which often reveals if there's a text signature (like OPUSHEAD in an Ogg Opus file) and didn't see anything. Frankly I'm unmotivated to do any more. They needn't have stuck to regular CELT as it's open source and patent free, and could have modified it anyway. Their software will know how to decode it, regardless of whether it's well-formed for a regular CELT encoder. I note that Mumble uses CELT internally and is used for in-game communications between players and the like. It is upgrading to OPUS, but might still support CELT. It's quite possible a game would base its audio around CELT for that sort of reason - compatibility with low-delay real time chat and high quality stored audio too in a single format.Maybe this gives you some more leads to try, depending how motivated you are to keep playing around, assuming you're allowed to do so.