Plantronics GameCom 780 under Linux, when using it as an analogue device has always been touchy for me; 1% volume is unusably loud for me. However using it as a digital output has always been fine in my experience.
Under Ubuntu 13.10 (and Debian unstable as of the posting date) the digital device is missing out of the box.
The fix is to edit /usr/share/alsa/cards/USB-Audio.conf, find the "Plantronics GameCom 780” 999 entry (line 46 on my Debian unstable laptop and Ubuntu 13.10 desktop), under the USB-Audio.pcm.iec958_device stanza and comment it out. This entry tells alsa that this device does not have digital in/out, which in my experience is wrong and renders the device unusable.
See launchpad bug 1241449 for tracking this.
Normal sysadmin related bloggery will return shortly.