No sound on my iMac except alert tones
I can't seem to hear anything on my iMac except alert tones. I was able to hear music and watch videos up until a few minutes ago. Now all I can hear are the alert tones. Nothing from iTunes or the web.
Has anyone had this problem before?
I recently upgraded my operating system on 9 July 2014 and have only just recently used the system again (this evening). I have upgraded iTunes in the hope that was a problem but no luck.
Any help would be appreciated.