As you can see at this link: http://xiph.org/vorbis/doc/vorbisfile/ov_open.html
Call ov_open loading it with LoadLibrary using visual C compiler/libraries may fail because of multiples CRT in memory.
The bug is in file dynamic_ogg.c because it calls ov_open.
I've made a small correction in my PC and it works now... the entire file is listed here. See the 2 #ifdef WIN32 blocks to see what was made.
Sorry about my poor english ... An as I'm new to bugzilla and SDL, sorry if I didn't submit the code throw SVN (I've never made this... if someone could teach me...)
[]s
Richard (Bigous)

/*
    SDL_mixer:  An audio mixer library based on the SDL library
    Copyright (C) 1997-2004 Sam Lantinga

    This library is free software; you can redistribute it and/or
    modify it under the terms of the GNU Library General Public
    License as published by the Free Software Foundation; either
    version 2 of the License, or (at your option) any later version.

    This library is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
    Library General Public License for more details.

    You should have received a copy of the GNU Library General Public
    License along with this library; if not, write to the Free
    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA

    Sam Lantinga
    slouken@libsdl.org
*/

#ifdef OGG_MUSIC

#include "SDL/SDL_loadso.h"

#include "dynamic_ogg.h"

vorbis_loader vorbis = {
	0, NULL
};

#ifdef OGG_DYNAMIC

#ifdef WIN32
/*
 * In Win32, dinamic link vorbis and call ov_open causes access violation is some circunstances.
 * It's better explained here: http://xiph.org/vorbis/doc/vorbisfile/ov_open.html
 * This is a work arround to make the SDL_mixer work correctly in Windows.
 * []s Bigous
 */

static int bigous_fseek64_wrap(FILE *f,ogg_int64_t off,int whence){
  if(f==NULL)return(-1);
  return fseek(f,(long)off,whence);
}

ov_callbacks bigous_callbacks = {
  (size_t (*)(void *, size_t, size_t, void *))  fread,
  (int (*)(void *, ogg_int64_t, int))           bigous_fseek64_wrap,
  (int (*)(void *))                             fclose,
  (long (*)(void *))                            ftell
};

int bigous_ov_open(FILE *f,OggVorbis_File *vf,char *initial,long ibytes)
{
	return vorbis.ov_open_callbacks(f, vf, initial, ibytes, bigous_callbacks);
}

#endif

int Mix_InitOgg()
{
	if ( vorbis.loaded == 0 ) {
		vorbis.handle = SDL_LoadObject(OGG_DYNAMIC);
		if ( vorbis.handle == NULL ) {
			return -1;
		}
		vorbis.ov_clear =
			(int (*)(OggVorbis_File *))
			SDL_LoadFunction(vorbis.handle, "ov_clear");
		if ( vorbis.ov_clear == NULL ) {
			SDL_UnloadObject(vorbis.handle);
			return -1;
		}
		vorbis.ov_info =
			(vorbis_info *(*)(OggVorbis_File *,int))
			SDL_LoadFunction(vorbis.handle, "ov_info");
		if ( vorbis.ov_info == NULL ) {
			SDL_UnloadObject(vorbis.handle);
			return -1;
		}
		vorbis.ov_open =
#ifdef WIN32
			bigous_ov_open;
#else
			(int (*)(FILE *,OggVorbis_File *,char *,long))
			SDL_LoadFunction(vorbis.handle, "ov_open");
#endif
		if ( vorbis.ov_open == NULL ) {
			SDL_UnloadObject(vorbis.handle);
			return -1;
		}
		vorbis.ov_open_callbacks =
			(int (*)(void *, OggVorbis_File *, char *, long, ov_callbacks))
			SDL_LoadFunction(vorbis.handle, "ov_open_callbacks");
		if ( vorbis.ov_open_callbacks == NULL ) {
			SDL_UnloadObject(vorbis.handle);
			return -1;
		}
		vorbis.ov_pcm_total =
			(ogg_int64_t (*)(OggVorbis_File *,int))
			SDL_LoadFunction(vorbis.handle, "ov_pcm_total");
		if ( vorbis.ov_pcm_total == NULL ) {
			SDL_UnloadObject(vorbis.handle);
			return -1;
		}
		vorbis.ov_read =
			(long (*)(OggVorbis_File *,char *,int, int,int,int,int *))
			SDL_LoadFunction(vorbis.handle, "ov_read");
		if ( vorbis.ov_read == NULL ) {
			SDL_UnloadObject(vorbis.handle);
			return -1;
		}
		vorbis.ov_time_seek =
			(int (*)(OggVorbis_File *,double))
			SDL_LoadFunction(vorbis.handle, "ov_time_seek");
		if ( vorbis.ov_time_seek == NULL ) {
			SDL_UnloadObject(vorbis.handle);
			return -1;
		}
	}
	++vorbis.loaded;

	return 0;
}
void Mix_QuitOgg()
{
	if ( vorbis.loaded == 0 ) {
		return;
	}
	if ( vorbis.loaded == 1 ) {
		SDL_UnloadObject(vorbis.handle);
	}
	--vorbis.loaded;
}
#else
int Mix_InitOgg()
{
	if ( vorbis.loaded == 0 ) {
		vorbis.ov_clear = ov_clear;
		vorbis.ov_info = ov_info;
		vorbis.ov_open = ov_open;
		vorbis.ov_open_callbacks = ov_open_callbacks;
		vorbis.ov_pcm_total = ov_pcm_total;
		vorbis.ov_read = ov_read;
		vorbis.ov_time_seek = ov_time_seek;
	}
	++vorbis.loaded;

	return 0;
}
void Mix_QuitOgg()
{
	if ( vorbis.loaded == 0 ) {
		return;
	}
	if ( vorbis.loaded == 1 ) {
	}
	--vorbis.loaded;
}
#endif /* OGG_DYNAMIC */

#endif /* OGG_MUSIC */