Oh yeah, that's the other downside to dmix: It doesn't support pause, so when you single-step frame-by-frame in mplayer, the audio gets ahead of the video and the video goes in a big burst when you unpause. mplayer could be smarter about this, but it isn't, and only works well with -ao alsa when HW pause is supported.
dmix could be a lot better with more special-casing for one stream. That would solve both of those complaints (for me, since I don't run any crappy desktop stuff that keeps a sound device open for your whole session. Nor any of the stupid audio players like rhythymbox that only has a pause button, not a stop button. let go of my audio device, you bastard...) That would make dmix a lot less sucky for something that's enabled in the default config.
SURVEY QUESTION: Do you usually only have a single client for your dmix pcm when you're not actually listening to two things at once? e.g. just watching a movie or listening to some music. Try sudo lsof /dev/snd/* (yes, you need root, because e.g. pulseaudio will have permissions you don't, so you can't access its /proc/pid/fd) If you see more than one process on /dev/snd/pcmC0D0p, you have more than one thing playing on device 0 of card 0, paused or otherwise.
Does anyone know how pulseaudio compares on any of this? I usually killall pulseaudio, but I haven't gotten around to weeding it out of the default X startup script, and it's part of Ubuntu's default install. I should probably just remove the package, since it's easy enough to reinstall if I want it. But I guess I keep meaning to check it out and see if it's better than dmix. So, is it?