I don't actually know how to tun off dmix! When I was using gentoo some years ago, to make dmix work I had to write a configuration file and put it into my home directory, called ".asoundrc". Since I've been using Ubuntu it "just works" and I've sort of forgotten the process.
The
dmix documentation has instructions to test if it's working, and change the configuration if necessary.
My dmix pointer is just a guess - I don't know that it's the problem. Check other avenues as well. One thing which just occurred to me is a simple check of the mixer settings - make sure your PCM and Master volumes are not at 100% - some soundcards can clip a little if one of the other volume's is too high.
For me, I get best results when my PCM is about 85% and master is no more than 90%.
Many apps in Linux will adjust the PCM setting when you change the volume in the app, but some do their own software volume control. In these cases, keep the volume away from the max setting too. In any case, I find putting the volume to max tends to cause some quality problems. Of course this is hardware dependent.
Another thing to do is find out what is your soundcard and google it with ALSA to see if there are any common problems and solutions for them.
Do you know your soundcard make and model (or better still, the chipset it uses?)