It looks like this will take a fair bit of rework of the music mixing loop.  Ryan, you want to tackle that for SDL_mixer 1.2.9?

Some info:

This thread on GameDev.net has some example code for looping to fill up the buffer from ov_read for seamless looping: http://www.gamedev.net/community/forums/topic.asp?topic_id=221938

It seems like that needs to be implemented in OGG_getsome(...)

One question is how OGG_getsome(...) should get access to the loop counter. The loop stuff is done at the higher level (checked in music_halt_or_loop), but for seamless it needs to be implemented at the buffer level.

I'd try to mess with it myself, but I'm not really sure how to go about tweaking the design.

this seems to make looping seamless for me, but i'm not sure how robust/solid it is.

hacky: not sure how to get at the real loop variable, etc. (always loops)

#define OGG_BUFFER_SIZE 4096
/* Read some Ogg stream data and convert it for output */
static void OGG_getsome(OGG_music *music)
{

	int section;
	int len;
	char data[OGG_BUFFER_SIZE];
	SDL_AudioCVT *cvt;

	int looping = 1;
	int total_len = 0;

	while (total_len < OGG_BUFFER_SIZE)
	{

#ifdef OGG_USE_TREMOR
		len = vorbis.ov_read(&music->vf, data + total_len, OGG_BUFFER_SIZE - total_len, &section);
#else
		len = vorbis.ov_read(&music->vf, data + total_len, OGG_BUFFER_SIZE - total_len, 0, 2, 1, &section);
#endif

		total_len += len;

		if(len <= 0)
		{
			if(looping==1)
			{
				vorbis.ov_time_seek(&music->vf, 0);
			}
			else 
			{
				music->playing = 0;
				return;
			}
		}
		else
		{
			break;
		}
	}

This is fixed for the next release, thanks!