You can try Adblock Plus or checking for malware.

• https://addons.mozilla.org/en-us/firefox/addon/adblock-plus/
• MalwareBytes' Anti-Malware
• TDSSKiller - AntiRootkit Utility
• Hitman Pro
• ESET Online Scanner

Did that. But still get sound from ads ... really annoying

There is no way you check for malware with 5 programs in one hour. Try resetting firefox.

To Reset Firefox do the following:

NOTE: Addons and some preferences will be removed, bookmarks and history will not be removed.

1. Go to Firefox > Help > Troubleshooting Information.
2. Click the "Reset Firefox" button.
3. Firefox will close and reset. After Firefox is done, it will show a window with the information that is imported. Click Finish.
4. Firefox will open with all factory defaults applied.

Further information can be found in the Refresh Firefox - reset add-ons and settings article.