Only one sound playback at a time

Only one sound playback at a time

For some reason reason I am only able to playback one sound at a time.
This means I can't do a Skype conversation and, for instance listen to
music streamed from Google Play or watch a Youtube video. I can listen to
VLC and YouTube or Skype/Teamspeak simultaneously.
When I open GNOME's sound applet and take a look at the applications-tab I
can see "ASLA plug-in [plugin-container]" and VLC media player as seperate
entries. Manipulating the ALSA plugin results in the sound-output of my
browser, Skype or Teamspeak to change.
Why is it that it is either Skype (or Teamspeak) or my browser that can
playback sound, but not all of them at the same time?
lspci | grep -i audio
Output:
00:14.2 Audio device: Advanced Micro Devices [AMD] nee ATI SBx00 Azalia
(Intel HDA) (rev 40)
01:00.1 Audio device: Advanced Micro Devices [AMD] nee ATI Cape
Verde/Pitcairn HDMI Audio [Radeon HD 7700/7800 Series]
Any help is greatly appreciated, since I have no idea how to troubleshoot
this.