There has been occasion where I've needed to convert MP3 to OGG. I know, I know, I know, it's never a good idea to convert a lossy format to another lossy format, but sometimes it's unavoidable. I came up with the following command to do just what I needed. I had to include --downmix and -b 32 for what I was doing, but they are likely not required. This just tells mpg321 to write to STDOUT in raw wav format, and oggenc to read the raw wave form from STDIN.

mpg321 input.mp3 -w - | oggenc -o output.ogg -