Comment 4 for bug 1388436

ubuntu only use 64k buffer

http://lists.freedesktop.org/archives/pulseaudio-discuss/2014-October/021998.html

sample spec: s16le 8ch 44100Гц
 channel map: front-left,front-right,rear-left,rear-right,front-center,lfe,side-left,side-right
              Объёмный 7.1
 used by: 0
 linked by: 0
 configured latency: 0,00 ms; range is 0,50 .. 92,88 ms

https://wiki.ubuntu.com/PulseAudio/Log

check whether you get underrun/overrun in verbose pulseaudio log

this 92.88ms may be too small for 7.1 audio