WebM does not contain ogg data, it contains vorbis data. Ogg is a container format, vorbis is an audio codec. FMOD cannot decode raw vorbis data, it can only decode it from within a container such as ogg or fsb (our own container format).
If you use libvorbis to decode vorbis to PCM you can feed it to FMOD piece-wise using the PCMREADCALLBACK.