Greenlife:
Applications > System Preferences > Sound > Output Tab
Get an old DVD/CD and put it into the machine. The media should play and sound should come from it. Or play something from iTunes. Remember to use the buttons for the sound on the keyboard to test as well. If you do not get sound at this point perhaps a driver was tinkered with and I would suggest the Apple store as they can fix this quicker then any other means. If you do get sound and the browser is at fault, check the youtube playback settings to ensure that sound is turned up and is not muted.
Truth
It sounds like the web has gained access to things it shouldn't on your machine. If you have a tech savvy friend I would take the computer to them and tell them that you were attacked by malicious browser websites. Identify which software you were using when this happened. There may be various residual issues outside of what you are experiencing now. Although MAC is better then most, it is not impervious to the web. Sadly the company does not acknowledge this. If a friend is not an option your best bet would be the apple store. Again, the simple issues you have mentioned do not mean that you are clear form malice.
If I have this issue and I am using a non standard browser like chrome or firefox I backup my favorites and do a scorched earth policy of removing the offending application and its files and then reinstall it. This does remove any saved passwords etc and history but it also removes any unknown content that could cause issues. In theory safari should be safer to use when "privacy" is turned on. Sadly technology like flash and pdf reader now allow for unsafe conditions to a computing system and cannot be trusted. And these are said to be the more safe plugins to have installed to your web browser. Even the bank vault is insecure if the door is wide open. Plugins although useful cannot be implicitly trusted. You machine is only as safe as the most insecure software installed to it.
Treating the symptoms does not prevent the cause
The only effective way to protect yourself I know of is to use an enclosure around the web browser and trick the system to believe that it has access to your computer. Rather like the reverse of the WW2 bomb shelter. VM-Ware, Zen, Virtua Box, all allow for operating systems and web browsers to be installed to a Virtal Machine (VM). By running the PC within a PC and only using it for web browsing the only thing that can be damaged is the virtual machine (VM). Since virtual machines (vm's) are nothing more then files on your computer they can be copied, deleted, and moved. Completely eliminating or vastly slowing down the ability for a virus or malicious site to infect your machine. This method allows you to use any operating system and any web browser you need to access the web.
By optimizing the VM and removing everything but what is needed to run the web browser you can get down to a smaller file size for the virtual machine and need not use 10GB. Once the VM is created configure the operating system and web browser to a point where you feel that it is useful and shut down the VM. Then look on the hard drive for the large file and compress it to create a copy/backup. Now when ever the browser is compromised in the future you can simply delete the VM file and uncompress the backup into the same location as the VM file you just deleted. Never taking more then 10 minutes of downtime, ever. Norton Ghost used to be my tool of choice in the Windows PC.
I am not suggesting that this is fun, but when you consider how many days this current inconvenience is costing you in time. 10 minutes reset time is irrelevant. Get the proverbial cup of coffee and talk to your office buddies giving yourself a 10 min work break, come back and everything is as it was before.
-light man
(Disagree? then constantly browse the dark side of the web and you will understand. Freedom is a belief, wisemen build their castle with a wall and moat. )