Sound troubleshooting

So you want your Linux system to play sound, but it does not? Follow these steps to analyze and solve the problem:

The Methodology

If your cables and volume are okay

• Test if your sound card driver and cables are okay; play a test sound:

speaker-test

If you hear a sound

If you hear a sound, your cables and drivers are okay.

If you do not hear a sound

If you do not hear a sound, see if you get an error message.

If you do not get an error message

If you do not get an error message, it can be a driver issue. To find out, delete all sound devices and re-create them:

tweedleburg:~ # rm /dev/dsp*
tweedleburg:~ # udevtrigger

Check again with dd if=/dev/urandom of=/dev/dsp. If you still do neither get an error message nor sound, it is most probably a driver issue. Get yourself a USB soundcard and proceed.

If you get an error message

If you get an error message like this:

tweedleburg:~ # dd if=/dev/urandom of=/dev/dsp
dd: opening `/dev/dsp': Device or resource busy

You should find out what process blocks your sound card. Do this with the command lsof (list open files):

tweedleburg:~ # lsof | grep dsp              
mplayer   18251       root    4w      CHR              14,35               14320 /dev/dsp2

You see, mplayer is blocking /dev/dsp2, you third soundcard. Now find out what soundcard you are using:

tweedleburg:~ # ll /dev/dsp*
lrwxrwxrwx 1 root root       9 Jun 21 10:38 /dev/dsp -> /dev/dsp2
crw-rw---- 1 root audio 14, 19 Jun 21 10:36 /dev/dsp1
crw-rw---- 1 root audio 14, 35 Jun 21 10:36 /dev/dsp2

/dev/dsp points to /dev/dsp2, so the soundcard you are using is blocked by mplayer. So, kill mplayer if you are sure that is what you want:

killall mplayer

Knoppix

When there is no sound under Knoppix try

/etc/init.d/alsa-utils start

strace'ing sound

I can hear sound on starting

speaker-test

and I can see with strace that speaker-test triggers activity inside the pulseaudio process. But how do they communicate with each other? First of all let's find out all threads of the speaker-test process:

linux-fpbq:~ # ps -A | grep speaker-test
29145 pts/1    00:00:07 speaker-test
linux-fpbq:~ # cd /proc/29145/fd
linux-fpbq:/proc/29145/fd # ll
total 0
lrwx------ 1 root root 64 Apr  4 15:44 0 -> /dev/pts/1
lrwx------ 1 root root 64 Apr  4 15:44 1 -> /dev/pts/1
lrwx------ 1 root root 64 Apr  4 15:44 2 -> /dev/pts/1
lr-x------ 1 root root 64 Apr  4 15:44 3 -> pipe:[114406]
l-wx------ 1 root root 64 Apr  4 15:44 4 -> pipe:[114406]
lr-x------ 1 root root 64 Apr  4 15:44 5 -> pipe:[114407]
l-wx------ 1 root root 64 Apr  4 15:44 6 -> pipe:[114407]
lrwx------ 1 root root 64 Apr  4 15:44 7 -> socket:[114414]
linux-fpbq:/proc/29145/fd # lsof +c 15 | grep 114414
speaker-test    29145             root    7u     unix 0xf5f19040      0t0     114414 socket
threaded-ml     29145 29146       root    7u     unix 0xf5f19040      0t0     114414 socket

ok, seems a socket that is open to speaker-test is also open to threaded-ml. So threaded-ml is a thread of speaker-test.

Now process 29145 is currently speaker-test and process 2289 is pulseaudio. strace'ing speaker-test with strace -ffp 29145 I find:

[pid 29146] recvmsg(7, {msg_name(0)=NULL, msg_iov(1)=[{"\0\0\0\0\377\377\377\377\0\0\0\20\0\0\0\0@\0\0\0", 20}], msg_controllen=24, {cmsg_len=24, cmsg_level=SOL_SOCKET, cmsg_type=SCM_CREDENTIALS{pid=2289, uid=0, gid=0}}, msg_flags=0}, 0) = 20

So speaker-test definitely gets messages from process 2289 and it gets them via file descriptor 7:

linux-fpbq:/proc/29145/fd # ll 7
lrwx------ 1 root root 64 Apr  4 15:44 7 -> socket:[114414]

Now strace'ing pulseaudio with the command strace -ffp 2289 I get:

[pid  2289] send(28, "\0\0\0\24\377\377\377\377\0\0\0\0\0\0\0\0\0\0\0\0L\0\0\0=L\377\377\377\377L\0"..., 40, MSG_NOSIGNAL) = 40

In other words, what pulseaudio sends via file descriptor 28 is received by speaker-test via file descriptor 7

linux-fpbq:/proc/2289/fd # ll 28
lrwx------ 1 root root 64 Apr  4 15:13 28 -> socket:[114415]

When restarting speaker-test with

strace speaker-test

I find a line

connect(7, {sa_family=AF_LOCAL, sun_path="/run/user/0/pulse/native"}, 110) = 0

and then file descriptor 7 plays the same role as before...

Look at man 2 connect and man 2 getpeername.

in the end solution was

linux-fpbq:/proc/32380/fd # ss -p | grep 186820
u_str  ESTAB      0      0      /run/user/0/pulse/native 186821                * 186820  users:(("pulseaudio",2289,28))
u_str  ESTAB      0      0                    * 186820                * 186821  users:(("speaker-test",32380,7))

See also

• stRace