Last time I checked TMPGEnc could handle OGM fine as a video source but not audio, even with an OGG ACM codec installed.
Although the above statement assumes that the audio stream(s) are vorbis. Also I guess TMPGEnc would have problems with multiple audio streams anyway.
Use VirtualDubMod to demux the audio stream you want, then decode it to wav with OGGDropXPd or whatever.
Then load the OGM as the video source and your wav as the audio source.
I guess you could also use graphedit instead of VirtualDubMod I wouldn't use mp3 filter though just leave the audio as PCM. Also if you are muxing the video into an AVI, last time I checked you had to have the avi mux interleave setting on full, either that or re-index the keyframes.