You can do it yourself, though it'll be a bit difficult. You'll have to extract the ogg audio, which can be done using VirtualDubMod, then convert it to something like mp3 using BeSweet or similar. Try to make the final size about 2 megs smaller than the ogg file if you want the avi to be the same size as the ogm. Once that's done I suggest using NanDub to combine the video from the ogm file with the mp3 you just created. It'll sound a bit worse since you're doing lossy->lossy conversion, but it'll be an avi.