Reference: safari 音が出ない 解決方法
I got the same problem that is written at the above blog. I realized that I can’t hear any sound from web, for example when I watch video on youtube. The above article describes how to fix that problem.
- Open Audio MIDI Setup (/Applications/Utilities/), then check the Audio Output setting.
- Change the Audio Output setting to 44100.0 Hz.
- Quit Audio MIDI Setup.
And he links to Apple support page for that problem.