The obvious thing to do is turn down the volume on the poker app. I assume it doesn't have a volume control, though. You may want to triple-check that. I'm afraid that beyond that, there's no hope. really:

There are multiple system volume controls. We're only interested in two: Master volume and "Wave" volume. Master volume is the volume control for everything that comes out of your speakers. It's the equivalent of the volume knob on your speakers themselves. "Wave" volume is the volume of waveform playback. Unfortunately, this encompasses both Winamp and your poker app. Winamp has a volume control of its own that is specific to Winamp and nothing else, but at 100%, it's as loud as the "Wave" and Master system volumes will allow it to go. Your poker app, if it does not have its own volume control, is playing at 100% loudness all the time. Which means that it's as loud as Winamp can ever be.

Too bad. Maybe there's some hack out there that will allow you to attenuate volume from just one app. I doubt it, but I suppose it's possible.