OGM is a container, Ogg may be a codec algorithm in it, MKV is a container, which contains, for example, an XviD stream. If Direct Stream Copy was enabled in VirtualDub (or it's mod that supports the Matroska container), then chances are that the quality wasn't reduced.
As for the audio tracks, they get direct-streamed or converted to WAV, so unless there's an MP3 codec specified, the music isn't irreversibly degraded.
If she overspoke (dubbed) the tracks, then I'm afraid it's going to be botulin doing the trick.

But the girls are not the only ones to reduce the lossy quality without a reason. For example, when I gave my wallpapers to the "Computer" magazine, they resaved them, sometimes getting a larger file than the original! 0.0 And at animepaper.net, I submitted a file that had 412k and they resaved it to have 387k! What's the point?!?