PulseAudio is the default sound server used in Ubuntu these days (including many other GNU/Linux distributions). Although I usually end up having troubles with it thus I just use ALSA instead. But many like it for its versatile features.
Anyhow, as a quick tip, if you ever encounter troubles with PulseAudio and wanted to restart the PA Daemon in Ubuntu or any distribution that uses PA, you can simply open your Terminal and enter the below command, and it should solve some of those issues (hopefully).
After issuing the command in Ubuntu you should see the audio icon changes into something similar as shown above. Just wait for few seconds and after the restarting process is completed you should see the default PulseAudio icon in your notification area again as shown below.