JACK не хочет работать

на вскидку, добавьте себя в группу audio и настройте /etc/security/limits.conf:

@audio - rtprio 99 # нормально -- 65, до 99, в настройке запуска jack ставьте значение на 10 меньшее чем тут
@audio - memlock unlimited # рекомендуют ставить половину от физической памяти, можно и всю (но это опасно если вдруг джек всю память съест)

мои настройки:

*               -       rtprio          0
*               -       nice            0
@audio          -       rtprio          99
@audio          -       nice           -10
@audio          -       memlock         unlimited

и попробуйте с realtime

Нет, не помогает

нет идей.

покажите тогда, что пишет джек при старте (в окне Сообщения можно глянуть для qjackctl)

00:07:55.274 Patchbay deactivated.
00:07:55.287 Statistics reset.
00:07:55.372 ALSA connection graph change.
00:07:56.981 JACK is starting...
00:07:56.983 /usr/bin/jackd -v -r -t200 -u -dalsa -r44100 -p512 -n3 -D -Chw:1 -Phw:1 -o2
00:07:57.028 JACK was started with PID=5173.
jackd 0.118.0
Copyright 2001-2009 Paul Davis, Stephane Letz, Jack O'Quinn, Torben Hohn and others.
jackd comes with ABSOLUTELY NO WARRANTY
This is free software, and you are welcome to redistribute it
under certain conditions; see the file COPYING for details
Memory locking is unlimited - this is dangerous. You should probably alter the line:
@audio - memlock unlimited
in your /etc/limits.conf to read:
@audio - memlock 1542591
getting driver descriptor from /usr/lib64/jack/jack_dummy.so
getting driver descriptor from /usr/lib64/jack/jack_net.so
getting driver descriptor from /usr/lib64/jack/jack_alsa.so
JACK compiled with System V SHM support.
server `default' registered
registered builtin port type 32 bit float mono audio
registered builtin port type 8 bit raw midi
clock source = system clock via clock_gettime
loading driver ..
start poll on 3 fd's
new client: alsa_pcm, id = 1 type 1 @ 0xee7360 fd = -1
Enhanced3DNow! detected
SSE2 detected
apparent rate = 44100
creating alsa driver ... hw:1|hw:1|512|3|44100|0|2|nomon|swmeter|-|32bit
control device hw:1
configuring for 44100Hz, period = 512 frames (11.6 ms), buffer = 3 periods
ALSA: final selected sample format for capture: 24bit little-endian
ALSA: use 3 periods for capture
ALSA: final selected sample format for playback: 24bit little-endian
ALSA: use 3 periods for playback
new buffer size 512
registered port system:capture_1, offset = 2048
registered port system:playback_1, offset = 0
registered port system:playback_2, offset = 0
++ jack_sort_graph
++ jack_rechain_graph():
+++ client is now alsa_pcm active ? 1
client alsa_pcm: internal client, execution_order=0.
-- jack_rechain_graph()
-- jack_sort_graph
5173 waiting for signals
load = 0.1378 max usecs: 32.000, spare = 11577.000
00:07:59.172 JACK connection change.
00:07:59.174 Server configuration saved to «/home/kronoph/.jackdrc».
00:07:59.176 Statistics reset.
00:07:59.179 Client activated.
00:07:59.191 JACK connection graph change.
Enhanced3DNow! detected
SSE2 detected
server thread back from poll
new client: qjackctl, id = 2 type 2 @ 0x7f888d979000 fd = 14
start poll on 4 fd's
server thread back from poll
new client qjackctl using 15 for events
start poll on 4 fd's
server thread back from poll
start poll on 4 fd's
server thread back from poll
start poll on 4 fd's
server thread back from poll
lock-driven null cycle
++ jack_sort_graph
++ jack_rechain_graph():
+++ client is now alsa_pcm active ? 1
client alsa_pcm: internal client, execution_order=0.
+++ client is now qjackctl active ? 1
client qjackctl: start_fd=7, execution_order=0.
client event poll on 15 for qjackctl starts at 3817905479
back from client event poll after 234 usecs
client qjackctl: wait_fd=11, execution_order=1 (last client).
-- jack_rechain_graph()
-- jack_sort_graph
start poll on 4 fd's
server thread back from poll
start poll on 4 fd's
server thread back from poll
start poll on 4 fd's
00:07:59.269 XRUN callback (1).
**** alsa_pcm: xrun of at least 25.883 msecs
client event poll on 15 for qjackctl starts at 3817966444
back from client event poll after 107 usecs
server thread back from poll
start poll on 4 fd's
server thread back from poll
start poll on 4 fd's
load = 0.6202 max usecs: 128.000, spare = 11481.000
load = 0.9648 max usecs: 152.000, spare = 11457.000
load = 1.0811 max usecs: 139.000, spare = 11470.000
load = 1.1004 max usecs: 130.000, spare = 11479.000
load = 1.2006 max usecs: 151.000, spare = 11458.000
load = 1.1774 max usecs: 134.000, spare = 11475.000
load = 0.9117 max usecs: 75.000, spare = 11534.000
load = 1.2397 max usecs: 182.000, spare = 11427.000
load = 1.1755 max usecs: 129.000, spare = 11480.000
load = 1.1304 max usecs: 126.000, spare = 11483.000
load = 1.1036 max usecs: 125.000, spare = 11484.000
load = 1.1074 max usecs: 129.000, spare = 11480.000
load = 1.1351 max usecs: 135.000, spare = 11474.000
load = 1.0973 max usecs: 123.000, spare = 11486.000
load = 1.1086 max usecs: 130.000, spare = 11479.000
load = 1.0022 max usecs: 104.000, spare = 11505.000
load = 1.0352 max usecs: 124.000, spare = 11485.000

дополнение: после изменений в /etc/security/limits.conf надо перелогиниться

в твоём примере не используется реальное время, попробуй с указанными настройками limits.conf и с включенным флагом realtime.