That is very odd... It chooses 4096 buffersize when using "-ao alsa" when it is playing both mp3 and video, but for some reason it works fine for mp3s and not for video. Since the buffer size is exactly the same for both, I would guess that is not the direct cause, but since the only change I made to cause that problem was to change the buffer size, the cause must at least be related. I will keep looking.