"Do you want to enable dictation" virus??

Remove Browser Pop-up Problems

Malwarebytes Anti-Malware 1.0.1.7

Adblock Plus 1.8.9, GlimmerBlocker, or AdBlock

Remove adware that displays pop-up ads and graphics on your Mac

How to remove the FlashMall adware from OS X

Helpful Links Regarding Malware Problems

If you are having an immediate problem with ads popping up see The Safe Mac » Adware Removal Guide, remove adware that displays pop-up ads and graphics on your Mac, and MalwareBytes for Mac. If you require anti-virus protection Thomas Reed recommends using ClamXAV. (Thank you to Thomas Reed for this recommendation.) You might consider adding this Safari extensions: Adblock Plus 1.8.9.

Open Safari, select Preferences from the Safari menu. Click on Extensions icon in the toolbar. Disable all Extensions. If this stops your problem, then re-enable them one by one until the problem returns. Now remove that extension as it is causing the problem.

The following comes from user stevejobsfan0123. I have made minor changes to adapt to this presentation.

Fix Some Browser Pop-ups That Take Over Safari.

Common pop-ups include a message saying the government has seized your computer and you must pay to have it released (often called "Moneypak"), or a phony message saying that your computer has been infected, and you need to call a tech support number (sometimes claiming to be Apple) to get it resolved. First, understand that these pop-ups are not caused by a virus and your computer has not been affected. This "hijack" is limited to your web browser. Also understand that these messages are scams, so do not pay any money, call the listed number, or provide any personal information. This article will outline the solution to dismiss the pop-up.

Quit Safari

Usually, these pop-ups will not go away by either clicking "OK" or "Cancel." Furthermore, several menus in the menu bar may become disabled and show in gray, including the option to quit Safari. You will likely have to force quit Safari. To do this, press Command + option + esc, select Safari, and press Force Quit.

Relaunch Safari

If you relaunch Safari, the page will reopen. To prevent this from happening, hold down the 'Shift' key while opening Safari. This will prevent windows from the last time Safari was running from reopening.

This will not work in all cases. The shift key must be held at the right time, and in some cases, even if done correctly, the window reappears. In these circumstances, after force quitting Safari, turn off Wi-Fi or disconnect Ethernet, depending on how you connect to the Internet. Then relaunch Safari normally. It will try to reload the malicious webpage, but without a connection, it won't be able to. Navigate away from that page by entering a different URL, i.e. www.apple.com, and trying to load it. Now you can reconnect to the Internet, and the page you entered will appear rather than the malicious one.

Not viruses. These are adware. They can affect any browser. These messages are automatically generated and not specific to any problem including yours.

OS X Yosemite: Use Dictation Commands to tell your Mac what to do - Apple Support

Bonferroni wrote:

When clicking anywhere on Google Chrome I would get redirects to a webpage asking me to install virus checking software. It didn't seem to happen using Firefox. I disabled all extensions in Google Chrome and it appears to have stopped.

That means that one of your Chrome extensions is adware. You can enable them one at a time, then test to see if the problem comes back, and that will help you identify which one it is. If you figure that out, it would be useful to post here to let us know what extension it was, so that others will know to avoid that extension.

However, I now get a popup box that says "Do you want to enable Dictation".

That's not due to adware or malware, that's just a feature of your Mac. There are shortcut keystrokes that can be accidentally triggered to enable dictation. The default is pressing the "fn" key twice, though it can also be changed to trigger when pressing the command keys twice. These are extremely easy to trigger by mistake, so I generally recommend turning this off if you don't use dictation. Go to System Preferences, click the Dictation & Speech icon, then select the Dictation tab. Turn Dictation off, if it's on and you don't use it, and change the Shortcut pop-up to "Off" to disable the shortcut keystroke.

Bonferroni wrote:

I ran the ClamAvx and got a few hits - all but one were from emails from several years ago and wouldn't have caused the trouble recently (the other one was an exe that I never clicked on - not sure that it would even run on the Mac).

The items ClamXav found would have all been either potential phishing e-mails or Windows malware attached to e-mail, from the sounds of it. None of those are a threat, or would have caused your problem. Hopefully you didn't allow ClamXav to remove those... doing so, with any anti-virus software, can corrupt your mailboxes.