Persistent problems: audio disabled, redirects and pop-ups on Google

Shortly before I bought Avast a little over three weeks ago, I noticed that, when I tried to change my speakers’ volume through the taskbar, I’d get an error message telling me there are no active mixer devices available – that I should go to Add Programs to add a mixer or somesuch nonsense. I already have a mixer installed! I researched the problem and none of the troubleshooting advice I’ve seen fixes the problem. After I installed and ran Avast, Malwarebytes and SuperAnti-Spyware, the problems disappeared until yesterday. Avast and the rest have all been updated daily, but the “no active mixer devices are available” trouble is back.

I don’t know if it’s related or not, but around the same time the aforementioned problem appeared, I saw that, when I did Google searches and I find the URLs needed, once I click on it, other windows popped up. Those were usually advertisements. I tried to close those windows, but a prompt – the sort of “For great savings, click OK to go on to X ad site” – appears that kept me from closing the window altogether. When I tried to click the red x on this prompt, it kept popping back up until I clicked on Cancel and/or closed the site through the Task Manager.
Well, THAT problem was back yesterday (the 21st), too :-\

What might this problem be? How can I fix it? Thanks for any help you can give me!

After I installed and ran Avast, Malwarebytes and SuperAnti-Spyware

Did avast, MBAM, or SAS find anything in your scans? Did your remove/quarantine them?

What kind of A/V - Security software did you have installed previously (if any)? Firewalls included.

Have you tried removing your sound card from the device manager, and re-install? It could be driver corruption.