Booted in the raring PC with the audio issues. dmesg output before reproducing the issue has the following line that looks suspicious to me:

[13828.248950] hrtimer: interrupt took 15288 ns

When started pulseaudio with the -vvvv option dmesg showed (did not open any audio program yet):

[27622.091908] hda-intel 0000:00:1b.0: Unstable LPIB (65536 >= 8192); disabling LPIB delay counting

After reproducing the issue by using mumble, dmesg shows *nothing* new.
At this stage, top shows:

nessita@dali:~$ top

top - 15:50:13 up  7:45,  8 users,  load average: 0.14, 0.22, 0.23
Tasks: 289 total,   3 running, 284 sleeping,   0 stopped,   2 zombie
%Cpu(s):  2.0 us,  1.6 sy,  0.0 ni, 95.9 id,  0.0 wa,  0.0 hi,  0.5 si,  0.0 st
KiB Mem:   8167864 total,  3780108 used,  4387756 free,   377504 buffers
KiB Swap:  6290428 total,        0 used,  6290428 free,   866680 cached

  PID USER      PR  NI  VIRT  RES  SHR S  %CPU %MEM    TIME+  COMMAND                                                   
11852 nessita   20   0 1461m  55m  32m S   5.3  0.7   0:13.38 mumble                                                    
10633 nessita    9 -11  330m 7100 4364 R   2.3  0.1   0:05.73 pulseaudio

Last few lines from pulse log are:

.... a lot ....
( 168.054|   0.000) E: [alsa-sink-ALC889 Analog] protocol-native.c: request_bytes(192)
( 168.054|   0.000) E: [alsa-sink-ALC889 Analog] protocol-native.c: ALSA Playback, pop(): 0
( 168.054|   0.000) E: [pulseaudio] protocol-native.c: Requesting 2048 bytes
( 168.054|   0.000) E: [pulseaudio] protocol-native.c: signalling underflow
( 168.059|   0.004) E: [alsa-sink-ALC889 Analog] protocol-native.c: ALSA Playback, pop(): 0
( 169.389|   1.330) I: [alsa-sink-ALC889 Analog] alsa-sink.c: Scheduling delay of 1.02ms > 0.96ms, you might want to investigate this to improve latency...
( 173.060|   3.671) I: [pulseaudio] module-suspend-on-idle.c: Sink alsa_output.pci-0000_00_1b.0.analog-stereo idle for too long, suspending ...
( 173.061|   0.000) I: [alsa-sink-ALC889 Analog] alsa-sink.c: Device suspended...
( 251.342|  78.281) I: [alsa-source-ALC889 Analog] alsa-source.c: Scheduling delay of 4.75ms, you might want to investigate this to improve latency...
( 251.409|   0.066) I: [alsa-source-ALC889 Analog] alsa-source.c: Scheduling delay of 12.15ms, you might want to investigate this to improve latency...
( 251.409|   0.000) I: [alsa-source-ALC889 Analog] alsa-source.c: Overrun!
( 251.409|   0.000) I: [alsa-source-ALC889 Analog] alsa-source.c: Increasing wakeup watermark to 16.00 ms
( 256.575|   5.166) I: [alsa-source-ALC889 Analog] alsa-source.c: Scheduling delay of 6.29ms, you might want to investigate this to improve latency...
( 256.591|   0.015) I: [alsa-source-ALC889 Analog] alsa-source.c: Scheduling delay of 5.32ms, you might want to investigate this to improve latency...
( 269.375|  12.784) I: [alsa-source-ALC889 Analog] alsa-source.c: Scheduling delay of 9.03ms, you might want to investigate this to improve latency...
( 277.341|   7.966) I: [alsa-source-ALC889 Analog] alsa-source.c: Scheduling delay of 6.62ms, you might want to investigate this to improve latency...
( 285.666|   8.324) I: [alsa-source-ALC889 Analog] alsa-source.c: Scheduling delay of 3.16ms, you might want to investigate this to improve latency...