Alsa has had dmix for quite some time. The thing was that until something like a decade ago it had to be manually enabled via a .asoundrc edit. These days Alsa will enable it automatically on any device known to not have a hardware mixer.
Maybe so, but I did try to fix it without resorting to a hardware sound card and couldn't at the time. IIRC the syntax of the ALSA configuration files was particularly arcane.
