This has happened to me on a number of occasions where I've found that my system audio only plays when I switch to a different tty. I don't know exactly how to duplicate the issue . But essentially, if I switch from my Kubuntu 14.10 KDE desktop to a different tty (using something like CtrlAltF5) and then switch back to the KDE tty, I find that I can hear no audio. However, if I switch again to the command-line tty, I can hear that audio that ought to have been playing in KDE. Rather weird, huh?
While I'm sure that there are cleaner ways to fix this, the one that I've found and use is essentially to restart the sound system using the following:
sudo alsa force-reload
This kills pulseaudio and restarts it.
Update: If pulseaudio does not start automatically for some reason, use:
This should get it running :) Hope this helps.