This is a similar problem to this post but I was unable to add to the comments since I am new to StackExchange. I tried the answer posted by Vishesh but it didn't solve the issue for me; when I run start-pulseaudio-x11 command it still says "Failure: Module initialization failed". However, the sound works for all my other applications (VLC, Rhythmbox, web-based audio such as YouTube and SoundCloud). I have not tried the solution posted by Vikas as I wasn't sure whether it would impact my applications that do have audio output.
The problem started when I switched my audio output to HDMI; I had plugged my computer into my TV.
Is anyone else experiencing this issue?