@ Necuno
If you return to nvidia-settings now that you have xinerama enabled there should be the twinview option available now (sorry i forgot that xinerama precedes twinview activation. Switch to twinview and save and merge to xorg.conf as done previously, this way if you don't like twinview you can easily revert back to xinerama with original settings as specified before. Xinerama seems to treat the 2 monitors as if they are one huge screen, twinview gives you 2 separate desktop spaces but you are still able to drag apps across to each fluidly, different desktop themes can also be applied to separate screens, comes in handy!
Also, to tweak your smplayer settings a bit, under the video settings:
Output driver should be vdpau
Enable post processing and turn quality up to 6
Deinterlace by default on Yadif (double framerate) or Yadif (normal)
DO not use software equaliser
Direct rendering on
Double buffering on (if it doesn't hog too much cpu then leave it on) can be monitored using ksys.
Do not draw using slices
Switch screensaver off
Everything else in video leave unchecked.
I'm not sure why this would be but now that your GPU is sorted you may find you no longer have alsa issues. If you do then describe what is happening and we can troubleshoot your alsa. Pulse seems to be ok, have you tried it out yet?